Think about how high your projectile will go. Two years ago the best trebuchet at Nationals ended up hitting the ceiling on 2 out of three shots. If trajectory is done indoors where you live you will need to keep the firing angle relatively low so you don't hit a ceiling. Consistancy is obviously the key, it doesn't make any difference at all in trajectory how for your projectile can go, only that it will go exactly as far as you intend. To get enough data points to be competitive at Nationals you will need to fire the thing over a thousand times. As someone said earlier in this thread, the winning trajectory device last year hit the nail at the center of the target on three out of the four shots. Keep in mind that the judges will probably say the ball hit the target at the center of the crater left in the sand, even though the crater doesn't form directly under the impact point (due to the angle of impact). You need to measure your impact point the same way the judges will (to the center of the crater in sand).
You also need to plan for the elevation of the target not being exactly what the judge (or the rulesbook) says it is. At our regional and state competition the targets were set up outdoors on somewhat uneven terrain. The event coordinator measured the height of the target from the grass under the target, not the elevation relative to the launch site. We had to do some quick and rough calculations of the actual height of the target vs. the launcher to get an accurate first shot. At states the event coordinator used an elevated tagret height that was not an interval of 10cm as per the rules, so we had no chart that we could use. So be prepared with a plan of how to deal with inaccurate target heights and distances as well as wind. I believe that this year you can only use a ping pong ball, tennis ball, hacky sack or racket ball (rules aren't final of course) so make sure you account for aerodynamics in your charts.
Of course most of these suggestions won't matter if you have a device that has a CEP (circular error probability) of half a meter. There is little reason to have charts with data points every 10 centimeters if your device can't consistantly send the ball into a 10cm circle. Even at Nationals, many of the trajectory devices had impact point spreads of 20+cm. Remember that the tie breaker is the distance between the first and second shots at the same target so even if you somehow manage to get one good shot at each target you could still lose to a team with a more consistant device.