Focus on a point ahead of you beyond the jump takeoff. It is important to focus on something and watch it spin all the way around, or else you will usually end up turning your head a slight bit a rotating. For example, if you are throwing a lincoln to your right, pop hard off the jump, looking ahead, throw your head towards your right shoulder to initiate rotation. To keep it straight, i actually throw my right arm behind my right knee. This keeps me straight and puts my hand in position for japan grab. Watch the world flip around, put the landing gear down and stomp that shit.
And as with most flips you will do for the first time, definately do it on tramps and diving boards first.