Anya has made some good points.
I have clients use the Abductor machine for a while when their knees come in. Usually there is an imbalance between Abductors and Adductors. I call it aerobic instructor's or runner's butt. They have stronger ADs and a cookie sheet ass along with other imbalances. Bad combo for deep squats until you resolve the imbalances.
Leaning forward is OK, depending on how your define leaning forward. You have to lean to some extent or you'll have problems out of the hole and knees will tend to come over the toes. Have to use a wider stance. I use a broom stick in the rack so their knees can't come forward. When they squat, they have their toes just to the point of the broom stick, forces them to use the hips. Best way to teach a squat along with a box for depth.
Good mornings, pull throughs, reverse back extn machine are all good assist exercises. Pull throughs especially.
I have also found ab/core work on a swiss ball to be very helpful. My clients that take my one hour core/ab class are far more stable with squats.
Regarding depth, learn to squat deep light before you start going heavy. If you develop bad form and compensatory neural recruitment patterns to get out of the hole, you'll be in trouble soon.
Lastly, you have to stay tight during the whole movement. I have some new clients that squat well, but turn to jelly on the last rep because they are fatigued. Then they try to speed up the last rep to get it done. Bad idea. Usually results in lots of yelling on my part. If your hands are on them at this point, you can feel it before they finish their last rep, I don't let them do another if they are not "tight".
Best thing is to have someone that really knows what they are doing work with you and watch you to begin.
W6
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