The most effective systems are a corrugated vinyl sheeting system that lay parallel across the top of the joists under the deck boards so the "troughs" of the corrugations lie between the joists and the peaks lay across the top. This requires taking up your deck boards and putting them back down, which is really only feasible if they are screwed and not nailed and your deck is not huge. It's a new deck so the fasteners should still be clean and easy to work. This both seals/protects both the tops of your joists and provides a water runoff path.

Next best is a hard lid-type system made of soffit panels attached to the underside. You can use actual soffit material, or corrugated plastic or steel panels that are commonly used for roofing/siding. Metal tends to span longer lengths than plastic materials without sagging. Install with the troughed side on an angle so that any water going through the deck goes into the troughs and then runs off the ends. Depending on how your deck is laid out and how much flow you expect, you may want to put a raingutter at the end to catch this runoff and funnel it aside to a specific point.

I'm looking at this second option as my deck is large and I'm not keen on pulling up the decking boards.