Seed mites are something that many seed savers aren't aware of but are very important as they can destroy your stored seeds very quickly.
In my experience they mainly attack brassica and cucurbit seeds the most and you should keep your seeds in clear containers (I use small plastic honey jars from Ebay, but jam jars are fine, so you can easily see the damage before it gets serious.
Seed mites are very difficult to see with the naked eye unless your eyesight is very good, but if you have them you'll see a fine brown or grey dust in the bottom of your jar. As soon as you see it you have to do something or your seed will be wasted. The two methods I use to kill them are 1, freezing the seed for a couple of days or so (never freeze your seed more than once, and never put back in the freezer if the seed is even slightly thawed), or 2, spray the inside of the jar lid with fly spray (if you aren't certified organic) and then shake the seed to give it a slight coat. The fly spray evaporates pretty quickly so you shouldn't smell it after a couple of days or so if you open the jar. I haven't found diatomaceous earth to be particularly effective, probably because they're too small to be affected by the sharp edges.
The main reason you get seed mites is storing your seed before it's fully dry. It will be dry enough to keep without rotting, but contains just enough moisture to feed the mites. This is why you should always wait another week before storing after you think it's dry enough. They can appear out of the blue at any time so always be careful and check your seed often.