It took me a lot of practice, but I love the Deck flip now. I disagree with a lot of the comments here. I find a newer deck is a lot harder to flip than a broken in one. I have also found that you don't need to toss your hand at all (it looks more magical this way too) Here's a couple of pointers that helped me.
Practice with the cards in the box first. I did this for at least 2 months. It gets you comfortable with the move.
Then use an old sticky deck. That way the cards stick together and won't have a tendency to fly everywhere.
Once you feel comfortable with a sticky deck, move onto a regular deck. Just put a little but of a bridge/bend in it to help the cards stay together.
As far as the actual technique itself, make sure your ring an pinky fingers are out of the way. I find I use the edge of my thumb knuckle, instead of the tip. If I was to simplify the explanation, all you really need to do once you've applied enough pressure with you index finger is let go with your middle finger, and then open your hand to catch it.
Hopefully some of this helps.