I'm a total dork, so I love
this kinda stuff.
Some things to keep in mind: 1) Circles/arches are fundamentally the "strongest" shapes you can use (as illustrated by that link), followed by triangles, squares, etc. 2) Contrary to popular belief, covering the toothpicks in glue actually decreases the structural integrity of the bridge and as a double whammy adds excess weight. 3) Make sure you reinforce the perimeter and ends of the bridge because that's where the majority of the stress is located in a traditional "flat" design. Hope that helps a little and good luck.