I find that this works wonders - "if you're not going to do it, just tell me up front instead of wasting both of our time"

When people are asking help from me, I try to tell them that they're either going to have to remind me at the moment they want it done, or let me get to it right away. I had a boss once who was simply bewildered by the latter - he'd assign me something, I'd jump right at it, and he was like "I didn't mean do it right now." He didn't realize that if it weren't done right away, and more assignments were lined up, it wasn't going to get done, no matter how much external organization existed.