How do I reduce the moisture under an exterior wood deck?
May I just add, providing spacing between the decking?  

When we moved into our home, the deck which has two sides connected to the house had some standing water beneath it.  The decking was pushed together and heavily painted, so there were no gaps for water to just drip off.  Instead it all flowed to the lowest corner and created a puddle underneath the deck. 

Once the decking was correctly spaced, water would drip evenly beneath the deck, now had a third option when evaporating which was up.