Gonen32: Mine are:
1. Buy games ONLY on sales.
2. Unless its a bundle, NEVER buy a game I don't plan on playing immediately.
The main reason for both is my huge backlog.
#2 is a new one for me, but I try to keep it. No need for more virtual dust collctors (sorry Doom for $2.5, you know I want you, it's not personal)
Wouldn't that imply buying only one game per sale? I mean, you can't play multiple games immediately. And buying only one game per sale would probably mean having nothing to play most of the time, unless you have a huge backlog.
I don't really have rany rules set in stone (other than the obvious no DRM, no early access or pre-orders), I just try take stock of my savings, my backlog and apply common sense. But yes, that does pretty much point to only buying games on sales.