Well - it depends..
Did you pull it after you cooked it or is it still whole?
If still whole then wrap it in heavy foil and put it in a 225 degree oven to get back up to temp.
If pulled and you want to do the while thing then put it in a foil pan and add a little apple juice, cover with foil - throw it in a low oven or back in your pit till warm.
I have heard of putting it in boiling water if you have it in a vacume sealed bag - not sure a regular zip lock would be a good choice I don't know if it will melt before the meat gets warm enough.
If you are just doing enough for a few sandwiches, then put it in a bowl, cover with a damp paper towel and nuke @ 50 - 60 % power for about a minute or so.