I read somewhere that the brain only has 7 active registers; you can only have 7 active subjects in your mind at any given time.
Therefore, if you are thinking about something, and then you purposely think about 7 new distinct subjects, the first one will suddenly pop out of your head without you even realizing it.
I've tried it before, and it seems to actually work. Only problem is that when you're done, you still have that sense of 'what was I doing this for again?', and you can sometimes figure it out and go back to the one you're trying to forget. Sometimes you get so wrapped up in subjects 5-7, that you forget the original one, and forget what you're doing entirely.
Now I can't remember where I was going with this.
GYRO adds: I guess I need a hardware update. Whenever I try to think about more than two distinct subjects, I get the natural equivalent of a stack overflow error.