I write it down.
Or I have a cup of Chamomile tea. Although its promoted as a sleep-aid, it doesn't actually have any sedative properties so its not dangerous. It does calm you though. A person I work with drinks it before giving presentations to settle her nervousness.
Another thing I do is ask myself, "If I wasn't so worried about X right now, what would I be thinking about or doing?" Then I turn my attention to whatever the answer is.