I like to put mine in an oven at 300. The time will vary with how large the brisket is. I would not slice it until you have warmed it up. We have reheated sliced pieces before, and they tend to dry out. Always keep the foil tight.
For smaller pieces, microwaves aren't bad. They heat quick and don't let the meat dry too bad. Just remember to use plastic wrap and loose the foil.