Might sound weird, but mentally extracting square roots does the job for me. Generally by the time the second or third decimal place comes around I drift off to sleep. Counting helps too. Count as high as you can on your fingers in binary. Use all ten and you can get to 1023. I generally don't make it past, or even up to 256.

If that doesn't work the iPod comes out and a few games of solitaire does the trick.

The whole idea is to shut down thoughts that keep you going - divorce, your ex, the child, etc.

Good luck, and though this seems trite, this too shall pass. It just takes time.