Set An Item Budget
This is something many people neglect to do. It's very important to decide what you are willing to spend before the bidding starst. To often you get caught up in the moment and end up spending way to much. I've seen $20 Wal-Mart Gift Cards go for over $25. People let the thrill of winning take over and end up spend to much. To avoid this adrenaline-induced high bidding, set a maximum bid (dollar limit) on how much you'll spend for an item--and stick to it! If the item is relatively common, bid a "maximum bid" and don't look back. If you lose, wait for the next one to surface and bid on it. The chances of your "must have" item surfacing on eBay again are pretty good. eBay is worldwide and people are always cleaning out their attics.
Set a Monthly Budget.
It's so easy to get your self into trouble with little $5 and $10 bids. Therefore, consider setting a monthly cap on your bidding and be vigilant about monitoring how much is left in your coffer after placing each maximum bid. Remember, you are obligated to pay the high-bid price for the item as bid retractions are rarely allowed. Don't risk not having the funds to pay for items won, if you can't pay then don't bid.
Over Budget
If you are close or have already gone over budget then stop. Close your browsers and turn off the computer. If you continue to look, you'll most likely continue to bid. Pay your sellers then take a break from the auctions. eBay isn't going to disappear over night so there will be plenty of goodies to bid on upon your return. Keep in mind that the majority of items you'll find on eBay become available on an almost regular basis. Vintage, rare, and out-of-production items will show up again. Turning your back for a while doesn't mean all the good stuff will be gone and never to be found again.