First of All, Your Late! SF and thanks. XP
I don't have a specific trick to it though because I just follow whatever I think looks good on it. I just tweak it based on light source and whatever Oda gives as folds for clothing.
but if you want to be technical about it, you can always try the law of gravity trick. shirt folds in a way where you can locate major stress points and then map out random folds from there or you can use references. Sorry not to much help on the subject. XD