If you are a broker/agent, make sure you register yourself and your clients online. Print out the confirmations. Bring copies of your license and your broker's license. And make sure you accompany your clients to the auction too!
If you're a buyer, you have to bring a cashier's check for $5,000 (or whatever the auction dictates) to be able to participate in the auction. You should also bring proof of funds. Don't forget your identification and personal checks. Make sure you take a look at all the properties you plan on bidding on a week before the auction. If you win a property at the auction, the auctioneers add a premium/fee of 5-10% of the sales price on top of your winning bid price. Keep that in mind when you are trying to stay at a certain price range.
Auctions are fun and great opportunities if you don't get carried away. Sometimes you can get better deals through a normal real estate transaction, so don't let the hype and excitement cloud your math skills!