Perfect Drywall Cuts
Here's a simple tip for cutting drywall. This technique will work for most straight cuts. With your drywall laying flat on the ground (grey side up) use a straight edge to line up your cut. Next, follow your utility knife down the straight edge, pressing just enough to cut the grey paper facing of the drywall (score the drywall). Once you've scored the drywall, stand it on end and fold it together like a giant piece of paper. When you fold the drywall, make sure you are folding inward from the opposite side of your cut. The drywall will snap perfectly where you scored it. Then, unfold the drywall half way and follow your utility knife down the inside angle of the drywall. The angle will hold your blade in place so you won't need a straight edge for this side.
It's important to remember not to press too hard with your knife when you score the drywall. If your blade is buried too deep in the drywall you will cause 2 problems. First, you'll dull your blades very quickly. It's very easy to go through a lot of blades when installing drywall because of it's gritty texture. Second, you'll have uneven cuts. Installing drywall with clean edges is imperitive to an easy mudding job. There's no reason to make careless cuts unless you're a master mudder.
If you are cutting a notch out of the drywall (a square out of a corner), this method will also work. The only difference is you will need to cut all the way through the drywall on one of your cuts. The second cut can be scored and snapped like explained above.
For more information about installing drywall, check out this article.