My goal is to get maximum force while avoiding pinching my hand between the cup and the lid, injuring a bone in my hand, pinching my fingers between the cup and the table, or having the dice fly out--all of which have happened to me at least once.
I deliberately do the opposite. First I gently shake the cup two or three times, just barely enough to hear the dice clinking as they hit the top. Then I put the cup gently on the felt, not making a sound. It is as if to make a statement that pounding the table doesn't help.
When I'm alone at the table, I almost never try to slam the cup. But if there are passersby or spectators, I will do it to try to draw attention to the game since most players don't know it.
When I'm in Macau, I feel the need to prove I belong at the table. I'm sure there are some underlying psychological issues I should look into, but really I just want to know how to slam the cup without injuring/embarassing myself.
My two cents on a technique for slamming the cup down would be place your palm on the very top and hold the bottom of the cup to the top with your fingertips, sort of like opening a big jar. My hands are probably larger than average, so if your fingers aren't long enough this may not work for you. But if you can pull it off you can slam the cup pretty hard with all the shock being absorbed by the meat of your palm.
All that being said, I have developed enough as a player (in my mind, at least) that I hardly ever slam down the cup. I realize now, like with many things in the casino, that although I thought it made me look cool to slam the cup as hard as I could, it probably just made me look like an attention-hungry goofball.