I mostly just buy games I like. I don't bother with CEs of games I won't play. I also try to give myself a budget and not go over it. Keyword: try.
But I always make sure to not go overboard. I don't have much time to go out, so I usually spend that money on video games. Sometimes it does get a little more crazy (like November), so I try to spend less the month after. I ask myself if it's something I REALLY need (some CEs are rather... lacking).
I use a credit card. I use it like a debit card online (as I can't use my actual debit card). If I buy something, I pay it off RIGHT AWAY (literally the moment it's paid for in PayPal). I only buy something if I can afford it. Like SquirrelRaper said, it also helps you build good credit! I've never been tempted to just buy something without paying it off. I tried once... ended up paying for it 5 minutes after. I'm not using my CC the way it's meant to be used, I guess!