As they cool, they naturally lose moisture*. I usually cut them into 1-bone segments then put a glaze of Sweet Baby Ray's on them (every surface), and put them in a foil covered glass pan. Sometimes I'll put a pan of water in the rack underneath...

A little fluid inside the container (glass/foil) with flavor tends to help mitigate that problem when reheating.

*the only acceptable use of the word, "moisture"