Since you do not know what the other bidder's highest bid is going to be, I suggest you resurch the item under the Completed Listings tab, under Search. Then bid your highest amount.
I usually need to bid more for items that are really hot, and less for items that have only one or two bids on them.
I also have my snipe ready with Auction Sniper, and then towards the last 30 minuets or so of the auction, I watch it to see how high it is going to go. This approch gives me enough time to go back and modify my top bid with Auction Sniper if it is something I really want, or a rare find.
Good Luck,
Laura