One of the things that I find helpful is to starch my fabric before cutting it. I find that it give me a clean, crisp line. The fabric is flat, and with the starch, the sewing line seems crisper, too. When I started doing this, I noticed that my blocks came out closer to the size they were supposed to be (at least a lot better than my attempts before starching). It seems that all my blocks are a little bit off, no matter how careful I am.
One of my bad habits is that when I get to the end of the seam, for some reason I don't keep a true 1/4" seam to the end. I don't know why this happens, but I try to pay close attention to this now that I realize that I do it.
I also use Arifil thread because it is not as thick as some threads and you tend to lose a teensy bit of fabric in the seams when it is ironed over a thicker thread.
In addition to this, I just saw a video (I think in was Nancy's Sewing) where they put a block of wood on the seam after they ironed it to weight down the seam while the fabric was cooling for a few minutes. The block of wood was maybe a 2"x4" cut about 10" long. Nancy said that it would help the seam lay flat.