Originally Posted by Genocidal
Edit: I keep thinking of new points
I'd add these points:
If a game and its sequel(s) is on Steam, it's possible you might be able to get better than 75% off individual titles by waiting on a bundle deal. Amazon is a good source for these.
Likewise, if it's a game with copious DLC, you might be best served watching for a bundle deal or holding out for a GotY version if it seems one is likely to be on the horizon. Some games like Magicka and Sanctum work out to be amazing deals when you buy their complete packs compared to what previous owners have to pay to get the pieces that have come out since.
The 75% off rule is a good one, but if you don't have many games on Steam yet, the Publisher Packs during the Summer and Winter Sales are a superior value. (Sadly, the last round of these featured incomplete publisher packs that left out games that had been included in previous sales, but they're still fantastic values.)