If you've been using Flash for any length of time, you're likely aware of the ability to use Layers and Layer Groups to organize and stack your content. Sometimes we overdo it on layers, though, and end up creating so many that it's almost impossible to remember what's where...and then we make a mess. So the question is: when do you use layers, and when do you stop?
Are you drawing moving objects?
If you're creating objects that move, then it's likely a good idea to keep each distinct moving object on a separate layer. One, tweens work a little funny if you try to tween multiple objects on the same layer. Two, you never know when your animated objects might overlap. If you're working with static artwork (such as the background to an animation), though, it's pretty much acceptable to keep everything on one layer.
Does the object overlap others?
Just like painting in real life, when you paint with lines and fills in Flash, overlapping paint on the same layer will replace anything it crosses. If you're drawing one object on top of the other and don't want to lose the full drawing of the first object, keep it on a separate layer so they can overlap without erasing each other.
Are you making layers for every last tiny thing?
If so, stop. Not everything needs its own layer. If you have a fill on one layer, its stroke doesn't need to be on the next. If being on a separate layer doesn't affect anything in your Flash artwork, then the object likely really doesn't need its own layer.