Thanks to Gary Ponzo, a fellow writer I met on Facebook, for this writing tip.
Because writing is such an empirical process it's hard to determine one tip which helped my overall skills. The only thing which comes to mind is the old line, "Show don't tell." And we all know to do this, but it's the one ligament which pulls at every aspect of my prose.
I realize it's a simple thought, but I'm a simple kind of guy.
(Comment by Dixon: A great reminder, Gary. Most readers enjoy solving puzzles, from big mysteries that act as the engine of the novel, to minor ones about what drives a character. Instead of saying some guy is a domineering brute, show him browbeating others in dialogue and through his actions. As Anton Chekhov said, "Don't tell me the moon is shining; show me the glint of light on broken glass.")