Good well-ground platforms below centerline are the key. You should direct most of the pressure inward, just enough to the side to pop the flake off after you have built up inward pressure. The books by John Whittaker (Flintknapping: Making and Understanding Stone Tools); and D.C. Waldorf (The Art of Flintknapping) are very helpful, as is sitting down with a good knapper and watching him work. There are also several good knapping videos on Youtube. Marty Rueter, aka Flintknappingtips, has a good one about pressure flaking an arrowhead from a flake.
