A good toothpick bridge design is one that has support in the most vulnerable places and can hold weight on all bases equally without breaking. Toothpicks are stronger when more are joined together to create thicker bases.
Sketch the bridge idea out on paper first to have a plan than can be changed if needed or scrapped if it doesn't work. A toothpick bridge made equally on both sides supports more weight. Toothpicks in an X shape give stronger support to the sides of the bridge. Enclose the X shapes with box shapes on either side of the X to make stronger sides. Thicker base legs hold more weight when bridges are tested.