As BJ mentioned, you gotta add the "buyers premium" to the total of your winning bid. Same here 10% cash another 3% if you charge it so mentally prepare yourself with a max bid prior to bidding and stick to it. You would not believe how many bidders get caught up in the moment and just want to win, regardless of what it costs. Some auctions offer an auction list, everything auctioned will have lot numbers and a be on the list. I walk around during the preview and make notes on what I am interested in, along with what I would like to spend and what I would be willing to go up to. If I get outbid on the first item, for example, I would be willing to go max on the next item but if I go max on the first, I may not pay max for the next item and so on.
Some bidders are talkers, when I go to an auction, I barely speak to my girl, much less anyone else there. It's strictly business with me, I'm not there to make friends. Some just want to know if you are interested in what they are, how much you are willing to pay etc, don't let them in your head and don't let them see your notes.
When I win an item, I usually snap a couple pics for reference so if anything comes up missing or damaged, I have proof. If you have several items in a row that you are bidding on, just write your number and the lot down and come back for pics afterwards, between bidding. I have even took pics prior to bidding just in case I get busy later but being somewhat superstitious, I think that can jinx you as well so be careful.
Lots of small things walk out, I have been to previews up to 3 days before an event and by the time of auction, something has disappeared.

I have heard of people swapping lot numbers as well, you win an item and someone wins the ****** one next to it, they swap tags and you come back to find you got got... Take pics, take notes and stick to the plans you made prior to bidding when you have more time to think.
Be courteous like Z mentioned, don't be that guy standing up front in the way when you don't plan on bidding on that item or the next few directly after it. I usually place myself on the back side of the circle, direct view of the auctioneers and group that is bidding so I can keep an eye on who is who(BJ's list of characters). Look the auctioneer in the eye when you bid, some people think because they raise their card, they won when you are standing behind them and the auctioneer knows you bid last(winning bid).
If there is anything you are slightly interested in but think it may go high, sit back on the sideline but pay attention, I have scooped up killer deals like that. Thinking an item will go high so I didn't even plan on bidding but when you hear "$10 going once, going twice" jump the **** in!!
Have fun and let us know what you win!
Edited: ALSO, if they end up bunching an item you are interested in with an entire closet, pile or shelf of other items that you are not interested in, pay attention to the winner, write his bidder number down and get with him AFTER the bidding, you may be able to purchase the single item you want for pennies on the dollar if he is only interested in the rest of the lot or other item in that lot.