Do it for the enjoyment of the competition. Have fun. Know that it is a hobby, even for the "Pro" folks, because very few (you can probably count these folks on a couple of fingers) make enough to actually pay the light bills and the mortgage payment consistently. The "Pro" folks are just more involved hobbyists...
Know there is no 1 type of BBQ contest. There are sanctioned contests and unsanctioned contests. Of the sanctioned contests, there are many sanctioning bodies that have different standards, different meats to cook, different definitions of the perfect BBQ, different judging criteria, different judges training, etc. etc. And the unsanctioned contests; anything goes for the most part. Just know that what works at say your local rib contest may or may not work in a KCBS sanctioned contest, and that may or may not work in a NEBS or GBA or FBA or MBN, etc. contest.
That's a long version of: READ THE RULES. ASK the organizer.
Then, practice, practice, practice. They all have a certain turn-in window. Practice cooking to that turn-in time. Right down everything you touch; that'll be your packing list. Plan on have water and power supplied at the contest, but also plan on the power going out for sometimes extended time periods and same with water... Me, I have a few lights to be plugged in for the night; otherwise we're completely power independent because it has always failed us... Water; largely the same. I truck in more than a few gallons, just in case...
If you can, attend a contest or two this fall, and dont just surf around. Bring a chair and sit back and WATCH some folks. If you can, apply to judge a contest. Seeing what comes across the table to be judged is very enlightening to many. Also look for example boxes (good and bad) on bbqcritic.com.
Know that it is a contest. You'll have a few teams that are there to party. The remainder of them, while they may be having fun, have brought their A game. It's on. They're there to win. Dont think that they slap a little KC Masterpiece on a grilled rib and put that through. These guys are serious and can produce some ****ed fine BBQ.
I remember coming back from my first judging experience saying "I've never in my 40 years ever, EVER had BBQ as wonderful as most of those entries". Think of the best BBQ restaurant you've ever eaten. For me, the very best I'd eaten was perhaps middle to bottom 1/3rd in that first contest I judged. And the best; well, that BBQ was as good if not better than mind shattering sex.
The reason I'm saying this is because of one of the teams I saw that first day judging. It was a group of young men, in their middle to latter 20's, they had an RV that they brought along. They were competing in an MIM contest where they'd cooked 2 whole shoulders and 3 racks of ribs. Total. That's the first major OOOOOOPS. Then, they got to drinking fairly heavily on Friday night. One guy stayed up, but apparently he fell asleep around 4. Someone finally woke up around 9... I was the first on-site judge to come see them at 10. It was a train wreck. They finally did begin to do a presentation and get me some Q, which wasn't done yet... It was mid presentation that of them asked me "How many judges do on-sites", to which I explained "I'm the first of three". That was when the "OH SH**" factor set in; a little late to realize that they needed to cook an absolute minimum of 4 shoulders and 5 ribs, and that's MINIMUM assuming they dont make finals. It was sad to see the sheer panic. I felt bad for them. They've never been back. For that I've felt worse. They didn't plan. They didn't ask.
Hopefully by you asking here that you wont do what they did, or frankly you will do what they didn't, and that's prepare. However, at some point, just do it. Jump in. You'll learn a TON from that first contest.
It can be a lot of fun.