In the short term, you should spend your limited willpower budget wisely. For example, if you do not want to drink too much at a party, then on the way to the festivities, you should not deplete your willpower by window shopping for items you cannot afford. Taking an alternative route to avoid passing the store would be a better strategy.
...willpower can grow in the long term. Like a muscle, willpower seems to become stronger with use.
In the short run, the strength of your willpower muscle is fixed, and you should use it sparingly. In the long run, willpower can be strengthened through more use.
This is interesting, and it may be right. But I would like to see a lot more empirical work before placing confidence in these generalizations. In particular, I would like to see clear definition and empirical description of the short-run, long-run distinction.
Thanks to Alex Tabarrok for the pointer, even though this is much more up Tyler's alley, as I pointed out here.