uhh. use a newer deck, and make sure you apply pressure. Use the PAD of your thumb.
as for 'licking it', i think it may warp the cards. plus it looks weird if you lick your fingers just for one flourish. (regardless of how cool it looks)
You don't need to lick your thumb there's already enough moisture. Also, push really hard to get it to stick and make sure you're pushing with the center of the card.