This happens to me so often I've lost track.
I have so many things to do. One thing gets to the frontal lobe (top of the heap, whatever) then something else jumps on top of it before I have a chance to start it. This happens a few times, and so I just tune out and start surfing FailBlog, playing Zynga poker, or something like that.
I've tried writing things down, but it just doesn't quite work. I always seem to fall short of sticking with something long enough to make it a habit.
If you've been there, how did you get out of that loop? How did you finally manage to get methodical about doing your stuff? Heck, how did you get effective at it?
Edit: Thanks everyone for your help. I enjoyed reading all of your answers.