Not trying to be mean, but you have some topology and proportion problems with that mesh and the thumb is all wrong. Rest your hand on your desk in a relaxed natural pose and look at which direction your thumb is facing.
http://www.youtube.com/watch?v=PTtk8Z_6AEQ&feature=relatedThis popped up when I clicked on "view in youtube" so I could watch it in full screen. It's done in maya and very fast, but most people should be able to get some useful information out of it.
http://www.youtube.com/watch?v=J1mExXURsWk&feature=fvwThis one is also Maya, but should help you with the thumb problem. It's a good start, but I don't really agree with everything the guys says.
