You probably have it too tight. Let gravity do some of the work and just let your fingers keep it from dropping as opposed to having it so tight.
Your hand does not need to be perfectly flat when you show it empty. Relax your hand and allow your fingers to curl in very slightly, this should look more natural. Just watch your angles.
Your thumb should be out of the way, but not sticking straight up. Also you can move your hand a bit. Display the empty hand, pause briefly and move on. If you wish, display the empty hand, pause briefly, move the hand a bit to seemingly give a better view, pause briefly and move on. One fluid motion with a slight pause for display here and there. I don't know what routine or trick you are doing, but a long, static display should not be necessary and it makes it harder to look natural.