It probably isn't my worst habit (on guitar), but lately I'm trying to focus on my expressional pauses. As someone around here has in their sig, the most important note is often the one not played.

I find I start out pretty good, but by 5 or 6 bars in, I'm trying to fill every beat then doubling on top of that, and so on. It's worst when I'm not using any kind of backing track. I feel compelled to fill in all the musical space.