Uh, if you are having difficulty getting the card to stay on your fingers during the flourish, try starting out practicing by doing the move with your wrist and thumb in a straight line. I believe in the videos that you may have seen people do it kinda casually on the table, in mid air and all that, but to start, keep your hand and wrist in a straight line and hold your hand downwards. (I'm not really sure how to explain this, hope its vivid enough). Essentially when you don't bend your wrist, you minimize the chances of you accidentally tilting/twisting your hand halfway through the flourish and causing the card to drop. Once you get a good feel of the balance needed for the card to keep spinning, take your time to try it at different heights and angles, and sooner or later you'll be spinning da bomb away with the cards.
With regards to the actual spinning, take trashman's advice. I had problem getting it to spin at the start because I was actually trying to use my finger to "follow through" with the spin and I was scared to let the card go. Once you've flicked the card, fluidly (but not suddenly) get your thumb out of the way and move your middle finger away from your palm. If you keep your thumb there there's going to be a lot of friction on the top of the card and you'll get a rather half-hearted spin from the card. If you don't move your fingers away from your palm, the card will spin one round, hit the inner side of your palm, then drop.
Happy learning!