How do you keep moisture out of an outdoor toolbox?

I have an aluminum toolbox in the bed of my truck and my tools are rusting. I need suggestions on keeping it dry. No rain gets in but it still gets damp. Here are a few ideas i thought up; blankets, baking soda, rolls of paper towels, and I even considered saving up those little packets that come in the jerky bags but that would take alot of jerky. Anyone else have this problem?