Go all the way down and all the way up for the full benefit of squats. Push your knees out as you go down, think about pulling your toes off the floor so the weight stays centerd on your heals and your not leg pressing the weight up.

When you get to a point below parallel you will feel a sort of bounce in your groin area and a faint one in your hamstrings, this is when you start going back up. Remember to drive your hips up, not out or forwards but up.

Imagine someone has a hook attached to the waistband of your butt area and is pulling it directly up. If your hips go straight up everything else falls into place.

Also make sure to look at a spot 6-8ft infront of you to keep your spine in alignment. And not up at the ceiling which has to be one of the worst practices ever. Hold your breath for the whole set aswell if you can as this will help keep your core string and tight(its more technical than that but I can't rememkber why) Only do it if you can though you don't want to pass out under the bar.