Yikes - the eyelids are a big subject! Here are some quick tips that come to mind:
2) Overdone Lids = Muddy Animation!
Remember, don't go too bananas with your character's eyelids. You want them to help strengthen the expressions and emotions, not distract from them. You're generally better off keeping them subtle unless the eyes are bursting open into a comedic wide-eyed expression or something major like that.
3) The lower lids are not used much!
There are times when you do want to animate the lower lids, and certainly emotions where you may raise them a tiny bit to help with the expression (suspicious, confused, etc), but other than that those emotion-specific instances, you'd probably only really need to animate the lower lids when the character is blinking (nowhere near as much as the upper lids, though), smiling (real, not fake), squinting, looking up (bring them up with the eyeball a bit), or possibly for anticipation to an eye-widening or change in thought process.
Hmm, now that I look at it, maybe they *are* used a lot! ha ha ha