I've been meaning to scan and post this forever...
Every good drawing book has a section on drawing folds. They're all pretty much the same. That said, the best one I've ever seen is from the "Famous Artists Course". Here is the first of several parts. Click to see these really big.
There are a few types of folds that are generally accepted among every artist, and the FAC lays them out just like every other drawing book, but for some reason the FAC version really seemed to be better explained than any other version I've seen. There are a lot of very clear visuals to explain everything.
This is the first section, which covers the basics, and probably won't strike too many people as all that extraordinary. The next part is where it really steps beyond what you usually see and really lays out a lot of good information I haven't seen before. I will scan that part when I get a chance.
I put folds in the same category as anatomy. You should learn everything about anatomy and folds and all the other nuts and bolts of drawing as early as you can; then these things become second nature and you can forget all about them, which means when you're drawing you can focus on all the really hard and important parts of drawing: getting the right pose, the right expression, the best composition and everything else.
Really, folds are pretty simple to learn and master, and the simplest suggestion of fabric and clothes is always the best, unless it's an important story point or something. The best drawn folds are those that help articulate whatever pose and/or feeling you're trying to convey and don't call attention to themselves or fight the more important parts of the drawing.