Tips for Steam Summer Sale?

the best advice is "not mortgage your house"

My tips.

1. Buy only if Daily Deal or Flash Sale
2. If 1, Buy if you will play it (save the ones you won't play for bundles)
3. If 1 & 2 Buy only at 75% off or better
4. Last but not least if all apply. Always check with your RU sellers

Days before the sale starts, I check for games I really want and put them on my wishlist (if they're not already there yet). Then I sort the games on the wishlist depending on how badly I want the game :)
When the sale starts, I will check my wishlist for discounts. Usually I'm fine with a 50% discount for games on top of the list. Most of the non-daily or non-flash-sale discounts last till the end of the sale, so I wait if a special sale happens. The graphic posted by AureasAetas sums this part up.

Tip especially good when it comes to indie games and people who don't have prices in $: if you're paying for games with real money, not trading/steam-walleting, BEFORE you spend money on Steam check official site of game maker - quite often they have same prices for games in that time.
And quite often in $, so if you have to pay in € you will save few more % due to Steam's $=€.
Just make sure they sell steam keys too (you might actually get 2 copies - Steam and DRM-free).

Both of you will be happy - you for saving few €, developer for getting more than 70% Valve gives them. Only sad one will be GabeN, but he'll get money from cards, so...

Use steamtrades instead of buying them directly from Steam.

dont buy anything if you have more than 5 games in your backlog.

Thanks for all the tips