I am a long time eBay Power Seller (and buyer), and the best advice I can give you is:
1 - Bid at around the 30 second remaining mark. I know that sounds like you're cutting it close, but that gives you the best possible advantage. Also, make sure your connection is fast (cable or DSL) for that time, because if its too slow, the server wont get your bid in time.
2 - If you really, really want the item, make sure you absolutely put in your bottom line bid at that mark - otherwise you wont get another chance to re-bid if you're outbid.
3 - Reiterating what another poster said, never bid an item up over the duration of the auction, you're only costing yourself money at the end.
Good luck!

A little tip, if you have a MyPoints account, you can place you're bid by clicking through MyPoints.com to eBay before you bid, and you get 2 points for every dollar spent! Easy Points!