Being well prepared for an auction is key. Sit down a couple days beforehand and work out your numbers.
Write down what you would be happy to get the property for. Then what your unhappy price would be.
Finally write down that absolute maximum price you would pay if it got you the property,essentially pretend it's the day after the auction, would you be gutted if you didn't pay X for it and it went to someone else?.
Keep that number in mind and don't go above that, at auction.