1. Is there any way you can reduce the weight of your structure without compromising the structural integrity? There are several ways to explore this:
a. Last year and this year, teams used 1/4 in by 1/4 in balsa sticks excessively for their structures. Can you use pieces with smaller (and lighter) dimensions?
b. As a followup, can you use pieces with smaller densities? Even if pieces have the same dimensions, they will have different weight possibly.
c. Consider also sanding a structure carefully or air drying the structure to knock off weight due to moisture.
d. Bracing. Will a Z-bracing work over an X-bracing? (you can possibly remove some pieces from the design and thus weight)
e. Glue. This is one of the biggest source of unnecessary weight. You don't need to apply a lot.
2. The way you load.
a. Some teams have the sand hit their hands first to reduce the force of the sand upon impact with the bucket. They usually increase their score by a couple ten's of a point.
b. It doesn't matter if the sand is level within the bucket. But do not load in such a way as to make the bucket sway. This introduces different directional forces to the bridge.
c. Consider placing the loading block in different locations.
3. Design Changes
a. Have you tried using gussets? Don't underestimate the value of them in certain locations.
If I think of something more, I'll tell you. This all said, the biggest help you can do for yourself is research. That's how the best teams did it. That's how we did it.