Go into the auction already knowing what you'll pay.
I realized that I tended to go nuts on auctions pretty early on and I've subsequently found 1 way to prevent over bidding. I look at the item and decide well in advance what I'm willing to pay. (Note: That's what I'm willing to pay, not what any price guide says it's worth though I may consult the book.) In the heat of the moment I'll sometimes go 1 or 2 dollars over, but not more than that. Yes, I'll moan and groan went it's something that I want really bad but I'll usually remember that set price and stick to it... no matter how much I don't like it at the time. I almost never regret it later whether I won or lost.
Oh, and I'm a college student so you know my collecting budget isn't huge.