Interesting perspective from David Allen, author of Getting Things Done.
If something is on your mind, Allen argues that it's almost always one of three things --
1. The desired outcome you want hasn't been defined.
2. The next action to perform hasn't been determined.
3. Or, you haven't set a reminder in an appropriate place you trust to let you know when it's relevant.
It's not quite a complete set of reasons something would be on your mind -- a lot of excitement about something coming up, being worried and not having the right techniques to dispel worry, or just turning something repeatedly over in your mind looking for solutions could all be reasons to think about something.
But it's an interesting very short checklist. I was meditating this morning, and a few thoughts kept resurfacing repeatedly. The still stood out to me after I got up, and I ran this check on all of them -- and sure enough, all of them were missing one of the three.
Definitely a useful diagnostic tool. Strong recommendation for GTD by the way -- a must read if you haven't read it, and a strong candidate for a re-read if it's been more than a year.