Sometimes it helps when I get into obsessive thinking to just write it all down as it crosses my mind. Not journaling really, just transferring it from my mind to the paper. And then when I get over the hump a bit rip all the paper up.
Lately I've had stray anxious thoughts pop right up as I'm falling asleep - usually I'm drifting off and then bam! "Omigod I forgot to do X at work they are going to fire me!" and I'm wide awake. What I've found really good for that is recalling some of my favorite fanfic or a favorite movie or something.
I do what Barb does, a lot. Pretty much before any kind of big conversation or major deal I've played different scenarios in my head.
I don't do it as much, but for the longest time my biggest outlet to deal with stress was just imagining - like writing fanfic in my head. I used to do this a lot - just pace in my room and make up scenarios and stuff in my head.