One of the latter is Allen's "two minute rule": if a task comes up that you think you're ever going to do (write a thank-you note, look up a reference, make a call), and if doing it will take less than two minutes, then you should always do it now. The rationale is that keeping track of it to do it later would take much more time than those initial two minutes, and delaying it will cause you mental friction in the meantime. If it's more than a two-minute task, then it's worth treating it as part of a longer-term system (which Allen also lays out) for keeping track of what to do when.
No kidding, Allen's book Getting Things Done is very much worth the money it costs to buy and the time it takes to read.
Now another useful-gimmick in the same vein: a way not to go crazy in dealing with email. The policy ls laid out here (and I learned about it here). Like the two-minute rule, it probably is impossible to observe in all circumstances all the time. And applying the hard-core version of this productivity strategy, laid out here, would probably make people think you are crazy. But the general idea makes good sense.