Flatties....
One important thing alot of people think. Because its called a flattie you are not LYING your bike flat. You are acutally levering it up into you!
Heres how i do it....As i push one hand into my armpit and the other one slighty twisted down in the opposite direction (towards the ground), lean down/forward into your bars as you do the motion (push and pull).
Just worry about doing this, it depends on the jump. Some jumps may cause the bars to turn a touch as you dig the grip towards your armpit (like me in the pic) and some will allow your wheel to stay completely straight (even with the frames downtube). Also keep your legs tucked in, knowbody likes a boner style bow legged flatty.
All i can say is, just worry about the techneque and it should all come to you, they do require some patients. hahaha i mean paitience!
Heres a picture of me doing one, i find it easy to pick up what im doing wrong by looking at pics of myself or otherpeople.