My favorite round-off progression has always been:
1) Handstand
2) Handstand 1/4 turn.
3) Handstand 1/4 turn, chest facing a wall. (gradually move in closer until starting with shoulder about 1/2 an inch away from the wall).
4) Lunge into 1/4 turn (basically, do a "handstand 1/4 turn" but don't let the hand closest to the wall touch the floor until the 1/4 turn is finished. If you are having trouble with this phase, it is likely the root of your round-off problem. An alternate drill is to do a "needle kick" and then turn your body.
5) Cartwheel near the wall.
6) Round-off (near the wall if on a soft surface, or on the floor in between 2 spotting blocks, or using a visual reference to stay in a straight line).
Some key things to remember, a round-off is moving forward and is a "twisting element." These skills must be done on an axis, therefore, your twist cannot begin until one foot is above your head, but before the handstand. It will help to look "down the road" towards where you are moving, and keep looking there until the second 1/4 turn. This means you should be looking under your arm-pit while you are inverted, in the direction you are going (not your hands).
Im sure other people will chime in with some things I missed, but this should get you off and running to cleaning your round-off up. I know another coach recently posted a great drill series for power round-offs with visuals and everything! Those might help too.
Good luck!
Ryan