I have really been enjoying playing with Google's fun FREE 3D drawing program called Google Sketchup! You can go here to find out more about it: http://sketchup.google.com/
After you download it, you can watch instructional videos that explain the basics of the program. Although I am only a novice at illustrations, I think it could be a great way to practice your skills! However, I have not been able to produce illustrations with it that have a good enough technical execution to be approved. But I still think Google Sketchup is a LOT of fun to play with!
(P.S. The image on the left is not mine, but I really liked it and thought it would be good for illustrating my blog.) :)