The amnesia rant won (by one vote) so here it is.
Note: This information is mostly on how amnesia can be used/misused in fantasy novels. Some of what’s said won’t apply to real-world situations.

1) If someone deliberately gave the protagonist amnesia, why?

The first question that’s on the tip of my tongue when I hear about bad guys trying to erase the protagonist’s memory is: Why don’t they just kill her? There may be a good reason why, just as there may be a good reason why the Dark Lord’s attack on the heroine’s family manages to kill everyone except the infant menace to him, (the one who should logically be the most helpless), or why the evil guys abandon the kid in the woods instead of killing her. But too often, the authors don’t give a reason. Other times, it may be “Insane Dark Lord mutter mutter.” I’ve already said that I think insanity is an overplayed fault in fantasy villains, and anyway, it doesn’t keep his saner lieutenants from disobeying him and doing the prudent thing. Please, if your villains want to inflict memory loss on the heroine, give them not one reason, but two: a reason for the memory loss in the first place, and a reason for sparing her life.
There’s also the fact that amnesia is chancy as hell. It may last months or years or even be permanent, true, but it may also only last a few days. The victim may not recall what events led to her getting amnesia in the first place—it’s pretty common not to—but even that memory might return with enough time and patience. In fantasy worlds where there are mages with some form of retrocognitive magic, which can see into the past, the danger of the secret being discovered increases tenfold.

2) Keep the circumstances of the memory loss as simple and non-contrived as possible.

No Rube Goldberg machinations, where “Oh, first she accidentally started a forest fire and then she rolled down a hillside and then she hit her head on a rock and then a moose fleeing from the fire stepped on her and then the villains couldn’t reach her because she tied them all up before she ran away from their camp and then…” ramblings are the order of the day.
One of the best memory losses I can recall reading is sustained by Morgon of Hed in Patricia McKillip’s Riddlemaster trilogy. It acts as a delay to keep Morgon in one place and let other people find him after a storm stops his ship, but no one bashed Morgon over the head in an attempt to keep him from talking; it came from an injury he sustained in the shipwreck. Even then, it lasted only one or two chapters, as far as I can recall, and didn’t drag on through the whole book. Morgon’s enemies kept trying to do what they were supposed to do, which was oppose him, not leave him an opportunity to escape their plans.
It’s possible to separate your character’s memory loss from enemies altogether, even though she might still have the bad guys hunting her. Perhaps she sustained a head injury in an accident. Perhaps she suffered from a disease or a sudden swift onset of massive pain, like a stroke, that damaged her memory. Perhaps she’s a telepath and memory loss is common to all telepaths after a while. (It never seemed right that telepaths, telekinetics, and so on could do their magic without paying any price at all). There are ways of getting amnesia into the plot and making recovery from amnesia a key part of a fantasy novel without unleashing the criminally stupid criminal bad guys on her tail.

3) People with amnesia do not generally forget everything.

If that was the case, then your amnesiac character has no business talking or recognizing the people around her as human beings. Most amnesiac people retain their command of language, and that’s not cheating in a fantasy novel, unless you’re going to say that everyone who loses their memory except this character forgets how to talk. (A character with amnesia and aphasia, on the other hand, would be a big damn challenge to work with).
So, it’s not a requirement that the character forget her name, her country of origin, her family, or her favorite color. It would be more interesting, I think, to read about a character whose rescuer didn’t have to give her a silly made-up name, especially if the amnesia lasted the whole book. (I am one of those people who couldn’t read Marion Zimmer Bradley’s Firebrand because she changed the name of “Cassandra,” the disbelieved priestess of Troy, to “Kassandra” and it just looked so wrong to me. It’s even harder when the character has gone by one name for most of the book and then the author changes things around). It also would lend more poignancy to the character’s story if she could remember her brothers and sisters as young children but had no memory beyond perhaps the eighth year of her life. This would make her someone with part of a past, instead of a great blank mystery, and might lessen an author’s tendency to take a condition like amnesia and run it into the ground with “drama.”
You may also have a character who recalls events from her past, before her amnesia began, but can’t forge new memories afterwards. Most authors seem to prefer not to work with them, though, probably because it would prevent that character from taking an active part in discovering what happened.

4) Unless the fantasy world has very advanced magic or technology, an instant “cure” is cheating.

This applies most in the cases of head injury, but also in the cases of fever, stroke, and so on. Above all, please don’t have characters assuming that all they need to do is bang the amnesiac character on the head again and everything will come back. How do they know this? They might as easily give the character aphasia, or blind her, or destroy what parts of her memory remain. And if they do have advanced enough knowledge of the brain to know just where to hit, I would expect them to have a solution more advanced than giving the character a tap on the head with a hammer or pushing her down a flight of stairs.
Likewise, be prepared to explain if it’s just a matter of having a mage or telepath look into the character’s head and discover where the answers are “hiding.” There are several major problems with this, not just one or two:

  • We are back at point 1 like whoa. If the character lost her memory to malice, then why didn’t her enemies anticipate a mage or telepath finding out the answer?
  • The memories may not be hiding. The memories may be destroyed. In the case of brain damage from a head injury, the likelihood would be much higher. It would be more worthwhile to have a mage go into a trance and reach back into the past, rather than trying to pick their way through the damaged landscape of another’s mind.
  • How does the amnesiac character know that the mage or telepath won’t damage her mind further?
  • If the cause of the character’s accident is supposed to be a mystery, and she won’t find out until later that her enemies are hunting her, then getting her memory loss reversed may not be a high priority. The person who rescues her might well assume the problem will reverse itself naturally. The rescuer might not have the money to pay for such a thing. The character herself may be content with the present situation, not in spite of but especially because she has no reason to think her memory loss is unnatural. (Also, see point 5). The mages and telepaths may live a distance away, or refuse to prioritize the character’s case because they have no reason to think it’s anything save a minor mystery. And so on. Authors actually cause more problems for themselves and not less when they make the loss so “mysterious.”
  • If memory loss around the event that caused the amnesia is total, the character herself has no bits of the accident floating about her head to urge her on. (See point 6).

Beware as well of the fortunate coincidence where the amnesiac character just happens to stumble on a mage who knows the rare and magical art of mind-healing, or a hedge-witch who has herbs that cure memory loss, or something similar. There’s a line I rather like that goes: “Each piece of literature is allowed one, and only one, fantastic coincidence.” Asking your reader to suspend disbelief by telling them the amnesiac runs by sheer luck into the one person who can help them is stretching it. And too often, something like this is merely a prelude to a string of deus ex machina happenings which can make the reader put the book down in disgust.

5) Why is the amnesiac character so fired up to get her memory back, anyway?

If she thinks she lost her memory in a malicious accident, then she might not want to remember who hates her enough to do that to her. If she lost it in an accident or disease, or what appears to be an accident or disease, then she has no reason to think it won’t return in time. If she suspects—and for most characters in a fantasy world, it would be rather a large suspicion—that she’s important enough to have hunters on her trail, then surely seeking help would reveal her. She might be well-advised to remain in hiding.
Basically, it’s not enough to turn your protagonist into an amnesiac. She must also be someone who is determined enough to find ways of restoring the memory, resourceful enough to persist in the face of discouragements, and has a reason to want to. If your character’s greatest desire is for a simple farming life, then why wouldn’t she just farm alongside her rescuer until her memory comes back to her? If she doesn’t remember that there are people waiting for her, why wouldn’t she find comfort with other people instead of agitating to return to her family? The problems increase because of her being in a fantasy world, not diminish. Most fantasy worlds have problems with travel and global communication. If the character was borne a long distance (as in a wrecked ship, a la Morgon of Hed), then it would be quite difficult for her family or lovers or enemies to find her.
Write someone whose personality will compel her to search out the truth behind the amnesia and work diligently to get her memory back. Don’t start with the amnesia and create your protagonist around that. It does no more good than building around a talent for fire magic instead of creating someone who could plausibly wield that fire magic.

6) Try to avoid the “somehow.”

You have to decide if the memory loss is going to be total or partial. Most important of all, you have to decide if the memory loss around the event causing the amnesia is going to be total or partial. Either way, try to get rid of “somehow.” This is the kind of thing where the character “somehow recalled shadowy dark figures threatening her,” then the next moment claims she remembers nothing about the event that made her amnesiac. Well, does she or doesn’t she?
One reason I get frustrated with “somehow” is because it’s the cause of much authorial cheating. It makes characters “somehow” know they have to follow people they’ve never met. It makes trainee mages “somehow” know secrets that other mages have to spend their lifetimes training to discover. And it makes amnesiacs “somehow” remember just enough to give off a vague sense of dread while not remembering enough to be useful. If the amnesiac remembers something about her accident or head injury or sickness or whatever, why does it always make her uneasy? Why is never something hopeful or neutral, or something her rescuers encourage her to interpret as hopeful or neutral?
Don’t avoid the work here, particularly if you plan to make the mystery behind the amnesia the true plot of the story. Let the character wander down some completely wrong paths, and don’t make them obvious red herrings. Let her recall things that will help her in concrete ways, instead of appearing and disappearing. Know the story yourself, so that you will know how to ease your readers and your character into the discovery instead of tossing out vague hints that could mean anything. (This is a problem that fantasy prophecies and broken memories share).
The technology rant is next.