It stinks to have a good cut of meat worthless the next day when you reheat it. However there is a really simple solution to keep your sliced meat and roasts from drying out.
When you store your leftover roast, be sure to place it back in its own juices. I know not everyone retains the juices after cooking a roast, but going forward do it, as it will keep your roast moister for when you reheat it.
Now, when you go to reheat your sliced meats place them in a dish with lettuce leaves between the slices. The lettuce will provide just enough moisture to keep your slices of meat from drying out. It is a really simple trick that works wonders.
