Hey everyone, I don't know about you guys, but I usually do a quick thumb first, dropping in some perspective lines, scan it, blow it up to size and then trace off onto good paper before going to my final drawing. Here's a tutorial I found helpful on setting up my perspective lines after I scanned in my thumb, and making sure what I had drawn was correct.
http://www.layerpaint.com/index_tutorial_detailed.php?roPos=1&catDisplay=1&id=2010#.WWY_S4jyu70
Excellent tutorial man, very helpful. Thanks so much for sharing this!