It seems like no matter what I try with clothing folds, even when I try hard to think about where they would appear, they always come out looking bad and....not like folds. Thus I don't put in nearly enough of them as I should.

Tips? Tricks? Treats?

I'm glad you want to know! It'll make everything look ten times better. Here's a good tutorial for that:

http://www.mangatutorials.com/tut/folds.htm

But the best thing you could do, is have a model! Wear a white shirt with a collar, and look in the mirror. Look at the folds! Real stuff are the best to referance!

Using a model is very key, but the main thing you MUST remember (and this goes for the ever-popular 'drawing the nose' question, too), folds do not have lines! Folds are simply sharp contrasts of volumetric tones (aka shades) that are close together.

Folds are 99% shading, 1% 'base sketch', so to speak. Alot of people have a hard time with folds, simply because they use LINES to convey them, when they should be using SHADING.

You know, unless the style you're going for is completely line-heavy, that's an exception. :D

But if you're going for 'realistic' or 'natural' folds, stay away from using lines to potray them. That link above is very good at showing the basic principle behind the fold, but if you want very stunning, beautiful results, be sure to shade, using the 'black lines' as the darkest contrast area.