The above advice is excellent. I can put a bit of perspective on things from Division 4. We're in our first year.
10 boxes is a good amount. We've done exactly that (with a little more for some training on the Saturday). Don't get all your paint at once, 3/4 boxes is probably enough. When you're getting down to your last box, send a runner or two for the next lot as soon as your next game is done.
We've used around 8 boxes at the last two events (and a bit, we've always broken open the 9th box and podded it up) and we usually take a box away with us. We've done ok shooting this amount of paint, 6th at our first event and 2nd at round 2. If you've got enough money for 10 boxes then you're probably sorted for paint.
Definitely make sure you have a spare gun made up. One of our guys switched his eyes off on his Etek and the rate of fire was set to 12bps. We're lucky they caught it as he was going on to the field as he'd have been ejected if it happened in game. We didn't have a spare and he had to come on the field without a gun. Not ideal!
Keep track of the games, the ultimate should give you a shout when you're next on the field so be kitted up and ready to go. Make sure your captain checks and signs the score sheet at the end of the game (I am TERRIBLE at this).
Be nice to the refs. It's a fairly thankless job. You'll get paint checked at the end of the point if you're a live player, a wee "thank you" or something afte they check you is always nice. They're out in the weather all day, we're lucky to stand underneath cover when it's miserable, the field refs? Not so much.
Speak to the vendors (you could probably e-mail them now if you haven't) and pick a reasonable grade of paint. One that isn't too pricey, but is fairly brittle and will shoot nice and straight. I took 4 bounces on the shin from a team shooting training paint on the day at round 1.
Above all, enjoy it. Our heads went down last round when we lost 2 out of our first 3 games. We got a nice pep-talk from a friend and ended up turning our day around. You're there to enjoy yourself and if you aren't having fun then you're missing the point. Learn from every win and every loss. A couple of games in start asking "what's working and what isn't?" Make small changes to your game plan (if you have one!) and make sure everybody knows what's expected of them.
Good luck!
