His rocket overwhelmingly won and was probably twice as fast as the next-fastest rocket, so we had something working out right. We "practiced" many times with our own at-home test string (highly recommended,) and we were able to perfect a lot of potential failure-factors along the way. Here's what we did that seemed to work out well:
1. VERY LIGHT ROCKET. I think that this is the key to winning. Make the rocket as light as you can while keeping it structurally sound. We turned it down on a lathe to about .125 in. and then carved out panels to .0625, leaving ribs and spars for strength. It was very strong and light. We also hollowed out every piece of plastic that we could hollow out to further reduce weight, even turning down the plastic "button" on the nose. We thinned out the fins to about a quarter of their thickness and made them very small (our rules required that you use all three fins but that they could be shaped.) Shoot for 11 grams or less.
2. SLIGHT NOSE-DOWN. Elsewhere, folks here discuss pointing the nose down to counter the auto-rotation problem that inevitably arises with a light rocket. We simply pointed the whole rocket down, very slightly. This kept the rocket from "looping" over the line. When the rocket cork-screws over the line, you lose big energy. Also, we found that adding counter-rotating fins didn't do much to ameliorate the torque-caused rotation. Tilting the nose slightly downward worked very well, though.
3. DRY RUBBER BANDS. As posted below, I'm convinced that soaked rubber bands rob the rocket of power.
4. CAREFUL SHAPING THE PROP. We shaved down a ton of weight from the first prop we used and this turned out to be a big mistake. By doing so, we removed some of the cant of the prop and it cut power in a major way. We bought another kit and put a new prop on and it made the rocket much, much faster. I would be very careful about reducing the prop at all.
5. CAREFUL THINNING THE TOP BRACKET. We over-thinned the top bracket to reduce weight and it caused the rocket to twist in the starting position, which then caused it to snap back and auto-rotate around the line worse than when we kept the top bracket thicker (and stronger.)
6. TEACH YOUR SCOUT HOW TO LAUNCH IF HE'LL BE LAUNCHING BY HAND. If you build your own line (again, highly recommended,) to test your rocket, have your scout practice launching his rocket without touching any part of it except the prop. If your pack has a "gate" then it's no worry. If you'll be launching by-hand, then the boy's launching ability will make a bigger difference than anything. We practiced about twenty times until he could launch the rocket without touching the rocket or without getting whacked by the spinning prop.
7. PAINT WITH MARKER. Paint can weight a LOT. Paint with Sharpies for virtually no added weight and a half-decent look.
8. MAKE A TOOL. If you have access to a lathe, you can turn down a piece of plastic round-stock to the inside diameter of the cavity of the rocket and then step it down to the nose cavity. By making a plastic rod that fits perfectly inside the rocket, you can turn it down to a very thin size without risking breakage. Also, the rod makes it easy to cut the fin slices without risk of crushing the rocket. It's a pain and not everyone has access to a lathe, but it sure makes it easier.
9. USE SUPER GLUE. As mentioned on this board, super glue worked very well for bonding parts and weighed next-to-nothing.
10. BUILD YOUR OWN TRACK. I can't emphasize this enough: practicing with your rocket makes a HUGE difference. I saw many rockets that night that were painstakingly built with great forethought but that failed for one unforeseeable reason or another. It was really sad. Setting up 40 ft. of tight fishing line is super-easy. You do have to buy the kit from the Scout House for a few bucks (to get the brackets that ride on the line,) but it's more opportunity to spend time with your boy "playing" with the rocket.
Thanks to all for your great advice! We had a blast with this.
Check out this thread http://derbytalk.com/viewtopic.php?f=17&t=1248
What I talked about in this thread you can get from the local hobbies shops in the valley. The best place to go is MRS Hobby located at 9860 S. 700 E #10 Sandy, UT 84070. That is where they sell the freeflight model airplanes. As with freeflight you will want to keep your weight as light as posable without having the rubber motor collapse the rocket body.