David Allen summarizes his "two-minute rule" as – if the action on something takes less than two minutes, do it as soon as you look at or think of it, 95% of the time you’ll save time and be much more proactive if you do it immediately.
The little, unimportant things too often demand much more attention later on than they deserve, and become too important because they weren’t handled when they would have been easy. And many of them can be handled very efficiently in the little weird windows of time we get in odd situations and circumstances.
From David Allen on Huffington.