Let me start out with the good, you really got the overall shapes right, you definitely got an eye for that 
But with that said, you need to spend a lot more time refining the shapes. What I think you need to do is slow down a little, from looking at your models, my guess would be that you jump very quickly on to the high polygon levels.
If that is a case, I think a lot could be gained if you took it a little more step by step, and started at the low subd levels for the major details, and slowly progressed on to the higher subds for the fine details and refining of the shapes.
What you got right now is some very bumby, not very natural look shapes/muscles skin folds etc., I believe that a lot of this could be fixed if you sat down and looked at some reference, and tried to think through, how does a skin fold look, what makes it look like it does, where are the underlying muscles etc. And I would properly even suggest trying to draw it.
Hope you can use some of this 
Like I said, I really think you got the over all shapes down really well, and it would be sweet to see you take these models all the way.
Keep it up mate 