Eh, I'm fine with backlogs since I can always just let a friend borrow it. Typically however I go by this rule:
If I already have a backlog, no full priced games until it gets low
If my backlog reaches 8, I can only buy games that are ten bucks or cheaper
If my backlog hits 15, then games five bucks or cheaper only
I won't allow myself to buy any games once it hits 20
Thankfully, I purchase carefully, so my biggest backlog is only the ~12ish ps3 games I picked up for ridiculously low prices