I'll stick to the "wait for black friday" strategy, normally they aren't immediately better than this one. And any sale like -75% or more or "no brainer" of that sort, I would still jump on.
For physical items, a lot of the better sales can pop up RIGHT before Christmas when the store is trying to get rid of their excess stock.
For software.... whenever it seems like a good deal, really. For instance, I will never buy Toontrack stuff at the regular price, so I just keep a list of the particular things I want and occasionally have a look around to see what they're going for.