We've used auction sniper for a couple of years now. You are only charged if you win the auction. You register and give them a credit card number and then you can put in the ebay number, make sure it is the exact item and then put in the amount you are willing to bid and when you want it to bid. It will ask if you want email notification if you are over-bid before the end of the auction.
I love it, as it means never having to worry about being around when an auction I am interested in is up and I also love that no attention is drawn to the auction until the very end, so the odds of getting into a bidding war is so much less.