this is for foil. . .
When you flick, make sure you are choosing that move, rather than just doing it because it works on beginners. If you get addicted to flicking you will get into big trouble when you find someone who can stop it! The main thing I think about is to raise your hand as you flick downwards. A high hand increases the probability of you landing the hit, and will also help you flick from a closer distance. With a high hand, and when I jump, I can flick someone on the back while we are standing almost close enough to be touching (yes I am bragging, but this is the only thing I am quite good at, and It's a good fencing party trick!!

). . . So a high hand is 5% of flicking, the other 95% is distance. Make sure you make the flick at the right distance! Otherwise you will either miss, or slash them on the shoulder. So that would be my advice. A high hand, and good distance.