I always find that one of the toughest things about sprites is getting them to look just right, and especially for animations. An example is this flying wasp-thing I made the other day:
There are a surprising amount of choice in how you can animate this. The way the wings move in all three animations are based on things I have seen done similarly in the past, but its always a bit of personal preference as to which style fits best. I like the first one the most but it certainly wasn't the one I originally tried (in fact they made in order from right to left). I find it best to try multiple designs before selecting which one. It sometimes leads to interesting points a long the way where I find aspects I like/dislike and incorporate them into the next idea.
On a side note it's really awkward getting iterative designs or work in progress because I always try to finish them up in a single session before getting around to copying earlier versions to a new file.