How do I make my trebuchet go as far as possible?

For a state competition I need to construct a trebuchet that fits into a 75 cm cube. At the regional competition we thought we did great until we relize that the tie breaker goes to the team the launches the farthest. So now that we are getting ready for states I want some advice on how to tweek it a little to go farther. The counter weight is unknown and will be between 1 and 3 kilograms. Same with the projectile unknown but between 20 and 60 grams. Right now it launches 3 to 4 meters. We were hoping to get it some wher between 10 and 15 meters. Any Advice?