Jack Reape has a great article on increasing squat depth – no matter which Powerlifting federation you favour. Amid the great advice is this :
I highly recommend a drill that my friend Pavel Tsatsouline teaches in his Strength Stretching video. It’s a squat performed facing the wall and should be done in front of a mirror, preferably in the cardio section of your favorite gym. Face the mirror with your toes an inch or two away from the wall in a wider than normal stance. Stick your butt back and your knees out and lower yourself as deep as you can go. Hold your hands out to the sides with your palms up to keep your sternum up as in the normal squat.
I like this one; though I’m at least spared the pain of having a full-length mirror in the home gym. Luckily a blank wall does the trick admirably.