I mostly used the normal round brush http://oi60.tinypic.com/16bmpz5.jpg. Instead of trying to make a good lineart (which I can't :P) , I used a color darker than the background to paint a rat silhouette, and I used the eraser to fix it. Then I added a lighter color with a lower opacity to give it volume (in this program, the low opacity also makes the brush softer, and the bigger the brush is, the softer it will be with that opacity), and I did the same with a darker color

http://oi62.tinypic.com/rkmxzp.jpg
I try to choose the color I want to use, instead of using a very light/dark color with very low opactiy, for that I use the color picker and then I change the B from the HSB http://oi60.tinypic.com/90qg7d.jpg, I only change the saturation if it looks too gray.
I made hard lines around the eye to make it more defined, and for the fur I made random lines(some ligther, others darker) over the shading. I think I also used the airbrush over the fur to add more shadows.

Very cool! I usually use a real light pencil because I like being quick and sketchy. I like the airbrush for the same way you use the eraser, but I'm going to try your method next. I was just thinking this morning if drawing with the eraser would be better.