Good advice. I'd also add a couple other points:
- Don't buy a game that isn't a daily deal! Unless it's 75% off (and even then) there's a chance it will be cheaper. Wait until the last day of the sale.
- Capcom and Activision are notorious for not having good discounts. If one of their games is 66% off or more, grab it because it may not be that cheap again.
- AAA or major publisher games generally don't drop below $20 non-sale price and $5 (75% off) sale price. If you see one at this price you want, grab it.
- Valve, Sega, THQ, SquareEnix/Eidos, and 2K are known for having very good discounts. Expect every Valve first-party game to be at least 75% off at least once during the Winter sale. Expect high profile 2012 games from the others to drop in price considerably (ex: Borderlands 2).
- Corollary: Sometimes the best deals on the above publishers' games aren't on Steam -- and that's fine! These publishers are all known for using Steamworks almost exclusively now, so odds are you'll be buying a key that activates on Steam. If you have any doubt though, ask in this thread.
- Indie games are usually a "bad" purchase. Many of them end up in indie bundles that provide a handful of Steam keys for anywhere from $1-8.
Edit: I keep thinking of new points