I did a project like this back in school along time ago. There isn't really a best way, but here are some helpful hints:
1. Try to be consistent. Make your bridge as symetrical as possible. For example, if you wnat to make a really simple bridge, say just 3 sticks glued to eachother in a straight line, dont glue stick 1 under stick 2, and stick 3 over stick 2. Glue both stick 1 and stick 3 to the underside of stick 2.
Not this:
___-------''''''''''' (stick 1 to 3, from left to right)
This:
______-------_____ (stick 1 to 3, from left to right)
2. TRIANGLES!!!!! Triangles are the strongest geometrical shape. More specifically, equilateral triangles. Imagine a sqaure structure with hinges at each corner. The square could easily mishape into a diamond at the hinges. Do the same thing (hinged corners) on an equilateral triangle (any triangle really), and you would never be able to mis-shape it without a break in the hinges or beams.
3. Take your time gluing. Glue can cause irregularities in your design. You want to add enough glue to make sure your frame is solid, but without mis-shaping your design. I recommend using just enough glue to get your overall design made. Once it is assembled, go back and add excess glue to each place you have one stick encountering another. This is a just a good method of ensuring your structure is well assembled.