We've only been to a few, but they aren't overly complicated affairs so I think I can share my experience.
Set your price
You hear this over & over again, but it's true. Pick your top price and do not go over it, there is always next time.
Do your research
My husband is a tool-nut and at the last auction we went to, I can't tell you how many times we had lots go by that he would comment that they were going over purchase price brand new at Canadian Tire. That is definitely not something you want to do. If possible get a list or even a general idea of what is going to be at the auction first, then do some research trips if you're not sure about what things should cost.
For the chicken auction, I had a list of what I knew I was going to be interested in. The good things about birds is you can always buy chicks, raise them up, and re-sell (hopefully at profit, depending on your management skills) if you decide later they aren't right for you.
$5 for Choice
This was a little confusing at first, but when large lots come up the auctioneer with auction of the chance to choose something out of the lot first, and you end up paying that price for every item you take. The first bidder usually pays the most, but not always.
For example: we bought at painting at $9 and choice and got to pick from about 20, the first bidder paid $20 to buy from 25 paintings.
For our ducks we paid $3, but I'm pretty sure everyone else who bought from that lot did as well.
Go for the big lots
For the birds, for whatever reason, individual birds out of the larger lots when for a lot less then the pairs or trios. I don't really know why this is, but I think it may have something to do with the number of people there obviously looking for pets.
Go Early, Bid Late
Do yourself a HUGE favour and show up before the auction starts. Give yourself some time to look around at what is available, and note the lot number and when it will probably come up. The first few lots to go through always go for the most money. People get impatient or just see the one or two things they came for, grab them and leave. It definitely pays to hang around until the end, the last few lots usually go for VERY cheaply when everyone already has what they want.
Yes, that can mean you're spending almost all day there, but you can save yourself some big $$ if you do.
Bring a Flashlight
I saw someone else with one and wish I'd brought one too! Smart! You need to check any animal over to make sure it's healthy and what you want. There isn't always the right/enough light in the hall.
Make Friends
Auctions are fun and usually full of people who share a similar hobby/interest! It's a great chance to network, as questions and meet new people.