Footage would be great. I have to re-correct this in my squat form periodically. There can be a number of issues that contribute to it:
- Bar too high or too low on the back: pushes out of the optimal bar path, and can cause excessive leaning.
- Knees traveling too far forward: pushes the hips up and the upper body forward.
- Weak upper back or core: unable to hold the weight up.
The best thing you can do while you are trying to fix this is:
- Use video from different angles
- Find the fewest number of cues that get you to the correct form
- Incorporate pause squats
- Think "heels, heels, heels"
Most recently the culprit has been my knees traveling too far forward. I've had to dump attempts on the safeties I would normally be able to make because of the forward lean. Pause squats, video, and a good forum of knowledgeable people are invaluable tools. A pause squat is where you stay at the bottom position for a few seconds, then explode out of that position. If you don't where squat suits, this has much more carry over to your unpaused squats than box squats. Also, you usually have a much better feel for what's going on at the bottom.
However, in your situation it could be that your upper back is rounding causing you to slump or do "squat-mornings" (combination squat and good morning) which is very hard on your body. If that's the case, you will need to strengthen your back to handle the load.