It is often talked about the weight being too far back, and that you wan't to move it forward with the hips. But beware of the upper body, if it moves too far forward, this can cause you to come in steep and hit it fat. This was one of the reasons, or should I say the primary reason, for me hitting it fat a lot. It is easy to try getting the weight forward by moving the upper body forward, but this can cause more fat shots.
On my second swing lesson with Evolvr, Dave addressed my upper body movement. I moved my head back on the backswing and too far forward on the downswing. First round I played after that lesson I didn't hit a single shot fat, shooting 78 vs 108 some days earlier, same course. It was incredible how effective that fix was. I have since relapsed a bit, got a little head movement today, but I'll give it some attention now.
The left knee is also something to look at. If it bends in towards the other foot, it compromise your balance point and can cause trouble. When I try to keep my left knee more stable, any movement being forward to add bend as the hips turn, my upper body stays more still and I hit it better. Look at this comparison between me and Charlie Wi, paying attention to the left knee. His moves forward, flexing more, while move moves in towards my right foot. When I try to emulate the left knee movement of Charlie here, my upper body stays more stable, and I hit the ball better.
To help you more effectively we really need some video, but you can of course try some stuff out yourself.
Here is a good drill if upper body movement is an issue: http://www.youtube.com/watch?v=F0qw0r7pDQU
Another thread somewhat on the topic: http://thesandtrap.com/forum/thread/43739/quick-question-about-head-movement-in-the-s-t-swing