Accurate cutting and squaring pieces up and then pressing your seams. I usually press to the dark side, though sometimes it is necessary to press the seams open so there is not so much bulk. Nesting the seams and pinning if necessary also help. Once you get the hang of it you can do without the pins. Let the seams meet each other and you should be good.
I have gone to using the Accuquilt system for cutting my pieces perfectly every time and that too has helped a great deal.