Wired magazine founder Kevin Kelly writes here about the "countdown clock" he created on his computer to remind him of how much (or, more to the point, how little) time he has left on the planet and that he shouldn't waste any. He's 55 years old, he says, so anticipates he's got about 8,500 days left, assuming he makes it to the U.S. male average for his age set of 78.63 years.
He reports that his friend Stewart Brand (founder of the Whole Earth Catalog), uses another approach:
"Brand, who is now 69, has been arranging his life in blocks of 5 years. Five years is what he says any project worth doing will take. From moment of inception to the last good-riddance, a book, a campaign, a new job, a start-up will take 5 years to play through. So, he asks himself, how many 5 years do I have left? He can count them on one hand even if he is lucky. So this clarifies his choices. If he has less than 5 big things he can do, what will they be?"
If you're tempted to set up your own countdown clock - and I am - then Kelly's article tells you where you can download the software. But if the idea freaks you out, don't do it. (Freaking out being inimical to productivity.) Keep in mind, however, that prioritizing is THE fundamental success skill, and if the countdown clock idea helps you do that, you may want to try to overcome any initial aversion you have to it.