I use them to strengthen lower back and stretch hamstrings, so I have a particular way I like them done.
If you are not touching the floor it is okay, bu you could get a lot more out of it by doing the following things:
1. Keep your scapula retracted at all times, in other words don't round the shoulders to get lower
2. Straighten your legs, and flex you lower back. Now push your ass back until you feel your hamstrings stretch.
3. Keep the bar touching you at all times, so instead of letting it drop straight down, you will pull it toward you as you push your hips away to allow yourself to go down.
This method will allow for maximum hamstring stretch and hence will place a ton of stress on the erectors during the eccentric phase. I typically do sets of 6 to 10 and focus more on improving back and hamstring flexability than weight. I use 315 for sets of 8 and even though I have good hamstring flexability this method only allows me to get just past my knees. I can touch my palms flat on the ground with straight legs if that gives you a point of reference.