My suggestion is not to display your individual cake slices as individual cake slices. Display the whole cake, pre-scored, and have the servers (assuming you have front-end people), slice it to order. The remaining cut sides of the cake should then have plastic wrap pressed neatly against them to prevent drying out. Keeping the cake whole is the BEST way to keep it as fresh as possible. Once a cake becomes a cake slice, the shelf life is shortened considerably.