Sliced brisket (especially the flat) tends to dry out fairly quickly when sliced so refrigerate or freeze it whole. Because I generally use the flat and point differently (flat for slices, point for burnt ends, chili, or sandwiches) I usually separate them before storing (if I didn't separate before cooking).
Reheat slowly to about 140oF (sous vide, foiled in slow oven, whatever) and slice/serve. You don't want it to dry out while reheating so wrap or keep moist somehow and don't reheat too fast.
