Perform Squats once a week in your leg workout the following way to achieve bigger and stronger legs :
Do 4 sets of squats 20 reps each.
The first set is a warm-up set.
The other 3 sets should be performed with optimal weight (go easy on yourself on the first time).
This exercise should be performed with a "pause-rep" routine. that means that you squat, take a few seconds of air, then squat again, until you finish your set of 20... as you reach the end of the set you will take more break of
inhaling between the reps.
Every week that passes you add 5 lbs more to your squat total weight.
Thats the way to advance :)
This method is perfect for drug-free trainees.