It's been a while since I've played with this idea. Greg Wilson did a bunch of work on this idea years ago.
A couple things that might help.
- keep your middle finger just in from the corner as opposed to on the corner
- practice with one card first, then work with two.
- practice with the card just an inch or less off your other hand just until you get the feel, then try at 4-6 inches away (enough so the card can flip)
- use a very light touch. Bend the card a small amount. Too much bend = too much force and the card has too much energy to control. The cards should barely bend really. Easy does it!