I could always get all the shots on the target, generally scattered about the target all inside about an 8 - 12 " circle at 33 ft. (This is all with a Beretta 92FS and I'm shooting right handed)
Here is what he told me to do:
1) I was standing wrong (with my right foot forward)
He told me to face the target, take one step toward the target with my left foot and lean on that left foot. My left foot should be directly under my head and in line with my left knee (head, knee and foot all in a vertical line)
Remember -- weight on that front leg -- not leaning back.
2) I was holding the gun wrong
Firm grip with right hand with thumb down on top of middle finger. Bring the left hand up and seat the heels of the two hands together -- wrap the fingers over the right hand fingers and do not try to use that grip on the front of the trigger guard as a place for the left index finger. He emphasized that point. The left thumb goes over the right one and does _all_ the work back there: safety, hammer cocking, slide release, clip release, etc. The right thumb should not do anything but hold the gun.
3) My arms were too stiff. I used to hold my right arm out fully extended and supported by my slightly bent left arm. He told me to relax and bend both arms a little as it is steadier. There is sort of a shock absorber function that way and all the motion of your body is not transferred to the gun. I found that it made the gun much steadier.
4) All the regular stuff about breathing and squeezing the trigger too.
Results: For me -- amazing. In that one session I tightened my groups (7 shot groups) from about 10 inches on average to 3-4 inches at 33 ft.