For this to be a real cost to the protag, it needs to be a mix of short-term and long-term memory loss. So, for instance, she should forget some sort of magical moment in her past with her husband, but she should also forget the mundane everyday stuff like an important appointment or the grocery list. I haven't fully explored this yet, but I do think one component will be that she'll know she's forgetting something but be unable to recall it. At least that seems right for the everyday stuff. I might make the long term memories more of a catastrophe, she doesn't even know she's forgotten the big important memory (e.g., wedding day) and that leads to problems in her relationships.

It's fantasy. It doesn't HAVE to be accurate, it is after all the cost related to an impossible magical power...however, I'd like to know more about memory loss and I think there are people here who have knowledge I can tap.

You might encounter an issue though, with regards to the limits of her memory loss. How long has she had these magical memory-draining powers? Do the memories come back after long periods of no use of magic? Potentially you could have a main character with no past memories because she's a heavy magic user and has essentially erased her past in pursuit of... well, whatever she's using her powers for.

All of this being said, I don't have any particular insights into memory loss in general, other than it is tragic. Was there specific characteristics of memory loss you wanted to discuss? You mention you would like to know more - what more, exactly? You're developing the how.

One common thing I have read is where you read a spell in a book and do the spell. The spell is suddenly erased from the book. One has to get a new book to cast the same spell.

One thought is that the person works hard to learn or develop a spell, then when the spell is cast, the spell is gone. One has to learn it all over again or never cast it again.
Magic school is learning the spells, thousands of them. The most powerful mages are ones who never use their powers. Those are the master teachers as they still remember all their spells.

Now consider a magician who is in a great war. AFter a period of time, he could use all his spells and be no more useful than a normal soldier. Magicians would be very expensive weapons.

As an alternative, There might be a super complex spell that uses just about everything a magician learns. It is, of course, so effective it will devistate an enemy nation.
The character understands all the stages of the spells. While individual spells would be forgotten immediately, she keeps reusing bits and pieces of that super spell to do everyday magic. Since she is using spell bits from within a spell, it is never lost, or so she thinks.
In reality, When learning a spell, which is more difficult than simply memorizing something, One uses a certain amount of power. Once a spell uses all that power, it ceases to exist in memory. The character using parts of a certain spell will use up the total energy contained in it and suddenly it is gone. Now a really imaginative character could hang a dozen similar spells, doing different aspects of the same thing, possibly repeating many of the commands in different places. That way there is enough spells for more than a lifetime.
In that last example, consider a computer program which deletes itself after it is used. Then consider a program with lots of sub programs. it can be used until all the sub programs or the total number of subprograms are used. Then it deletes itself. Also consider the possibility of writing the commands that will cause it to copy itself before it reaches a certain point so that it can be used some more, if not completely.

And yet, scientists appear to have discovered a technique for reclaiming lost memories:

http://www.independent.co.uk/news/science/scientists-discover-way-to-reverse-loss-of-memory-775586.html

So, putting these ideas together with some from previous posters:
When MC casts a spell, she forgets part or all of it. But the memory is still there, and connected to other memories not forgotten. If she searches her mind, or remembers memories related to the spell, she can reclaim the memory of it.

Suppose she has a vanishing spell that involves crushing the eye of a newt and throwing the remaining mess in the direction of the person she wants to make disappear. If she vanishes someone, she forgets how to vanish people.

If she then wonders, "How can I vanish a second person?" she cannot remember the spell. The spell hasn't exactly disappeared from her mind, its memory has become disconnected from the words "vanish someone."

But if she discovers a newt's eye in her pocket--carefully secreted there before casting the spell--she remembers that eyes of newts can be used in vanishing spells.

I first thought of the movie Memento If you have not seen it, I would recomend renting it before you delve into this wonderous story. It is a story of a man with amnesia, that is involved in a deep plot, but has no recollection of the previos days events. So when he wakes up each morning, he has no idea of yesterday. He starts to leave himself clues. Symbolism was important--associative affects.

As you mentioned, the "cost" needs to be worth of the "cast" so to speak. In order to be effective, it has to be something important to the character.

Without knowing much of the plot, my first instinct is to have the character forget much of what they are doing so far as what is driving them through the plot--their goal. For example:

Let's say that the character is trying to prevent the MC from finding out that he/she is magic born. The MC is on a quest to wherever to find out whatever about it. As the antagonist the character would no doubt be trying to prevent that. So what if the caster had to awaken every morning read through all these notes and clues to even know what they were doing, or where the MC might be.

So far as conveying the memory loss, it is imperative that it be done in a way where it is understood what is happening. It would be hard to pull this off in the POV of that character because the reader would no doubt be as confused as the POV character. Pulling this off will prove to be the biggest challenge I'm sure.

I have four hundred books in my personal library and they are filed according to how quickly they could be stacked into the shelf after the previous one collapsed. It would take time to find it, if I could recognize the title or spine.

He does have the power to see magical things even if they are invisible. He can see the gods as an example.

At one point, he is thinks he is lying about what he wrote in the book in previous times. They are too fantastic to be real. An associate asks if he lied in his journal today. He says no. then it is likely he did not lie in the previous recordings.

And paranoia! Awesome. I'm looking for ways to set up conflict between her and her husband. Other than something horrible happening to her children (which is something I'm just against doing in the fiction I write because as a mom I find it way too unsettling to even contemplate) the main conflict/point of tension/thing the MC has to lose/thing that is at risk is her relationship with her husband. Paranoia gives it quite an interesting twist...

Just to provide info for any who were curious - this is my Teleport This story that was in F&F a few weeks back (yes, I take forever to finish stories...I'm a mom, I write in 12 minute increments on every third Thursday afternoon and alternate Wednesdays between the hours of 12:01 AM and terminal eyedroop - which is about 12:07, and ... LOL, you get the idea.) The MC is a suburban mom who discovers that she can teleport the minivan anywhere she wants. But there has to be a cost, or she (and the other people like her) would just teleport everywhere. So, it's a new skill, she hasn't yet figured out the cost. There's all kinds of potential humor in the cost of memory loss. I'm playing it that the cost is that using this new magical skill ages the users - some get gray hair, some wrinkles, my MC will start to lose memories.

But you might also consider the memory lost being roughly equivalent to power of the task. Something small like, say turning the oven off when she smells smoke, for example, would only make her forget something of the grocery list, but putting out a house-fire would make her forget her wedding day.

Or maybe even switch it, so that when she feels strongly about the NEED to do something, it has a smaller cost. But if it is a casual or even lazy use of magic then it draws a bigger cost (maybe because the magic is semi-sentient and doesn't like being misused?)

Just thinking "aloud" here. Be interesting to see how this turns out, I look forward to it.

Edited to add: I check things at Amazon, to make sure I remember correctly. Didn't check ahead of time with the Sanderson books, though. According to Amazon, the series is named MISTBORN and the first book is named THE FINAL EMPIRE. Which is not how I remember thinking of them. <shrug>

The third book will be titled THE HERO OF AGES, by the way.

While I was still having a severe problem with short term memory, I would get very anxious about whether or not I would be able to find my way home from work in the evenings. n order to ensure that I would be able to get home, I'd write myself a note with directions and pin it inside my shirt.

Often, in the evenings, I would stay late at work 'cause I had no idea where I lived and how to get there. I'd site down after everyone else was gone and try to figure out what to do. Eventually I would realize that I would have known this could occur and would have written myself a note. I'd search my clothes and find the note, and get myself home.

For example, I could lose my memory of my first grade teacher's name. In that case the name is gone. If you told me "Her name was Mrs. Johnson" or "Her name was Mrs. Kennedy" I would have no way of distinguishing which was correct. This has been used by many fiction writers.

Less frequently used is a loss of recall; the memories are there, but the character cannot control access. Suppose your character has a Hatrack River Writer's Forums password. While she is driving to the mall she can think of the password, but when she needs to log on she can't. Maybe she asks her memory for the password and instead it serves up the theme to Gilligan's Island.

Because this involves magic, the memory scrambling might be specifically targeted to impair her ability to achieve plot-related goals. Perhaps she has an excellent memory, she can always recall her first grade teacher, but at a critical moment she cannot recall what sort of car the villain drives.