+1 for emphasizing contrast. It really is one of the most important things in pixel art.
Here's a sprite I made a while back. My final version was the second from the right, then a friend of mine recolored it. Pretty big difference, eh?
<img src="http://dl.dropbox.com/u/17634050/Mech_Stages.png" border="0" />
What I'd really love to see is more tutorials on STARTING a sprite. Once I have an outline or rough figure of what I'm going for, it's pretty easy to finish, but I find getting that outline in the first place to be really difficult.