I had been working with Blender for a few weeks when I finally said, out loud, to my computer screen, the following:
Why do my meshes always have horrible creases?!
The answer was: I wasn't paying attention to topology. So here's some links on that subject:
A description of the basic problem of triangles and other mesh issues. (Actually Geurillacg.org has loads of awesome videos.)
One artist's methodology for keeping meshes sane.