I recently bought the book "How to Draw" by Scott Robertson and worked through it. It was full of great excersizes, tricks, advice and insights. I highly recommend the book to anyone interested in improving their craftsmanship.
So, just to spread the word, you can find out more about Mr. Robertson on his youtube channel (which also has lots of great tutorials).
www.youtube.com/user/scottrobe…
Happy drawing!
I just emailed my entry for the Unofficial Marauder redesign contest on the forums of Mechwarrior Online. It was a fun run working on this one and it really pushed me. There are some very good artists like ShimmeringSword posting their art in there and I highly recommend checking it out.