Distractions.

The internet[1] being the major one, occasionally video games or books. Somewhat unpleasantly I seem wired to seek new distractions whenever I don't have one currently occupying my mind.

I've been thinking of forcing myself to spend half an hour each day without stimulus, just thinking and writing on a paper notepad to get around this.

[1] I find posting on the internet much more distracting than surfing, because there's the constant desire to check back and see what responses you got.