Apple juice. Plenty of apple juice is what I use.
If I'm pulling pork ahead of time with the intent of chilling it and reheating, I'll douse it with GOOD amount of AJ before putting it in the fridge. Then as it reheats (covered) the AJ keeps the meat hydrated. If held in a chaffing dish, crock pot or roaster, same thing. Just put AJ in the meat and stir it often to prevent drying it out. Check often and if it's looking dry, just add more AJ and stir.
Works for me at least and noone knows any different.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team