Work at it on a gentle green hill, keeping your base dead flat and your body aligned dead straight over the board. Bent knees, no rotation. No cheating with subtle edge pressure. You'll feel the board get squirrely under your feet, shimmying back and forth. It's unnerving, but if you maintain your composure and stance, you'll reach a point where you get comfortable with this. The subtle oscillations will self correct. You'll learn at what point the board kicks too far out of line and slams you, and when you need to consciously correct. Sometimes (usually the faster you go), it's just too late.
Whatever you do, MAKE SURE YOU KEEP IN CONTROL AND CAN STOP OR AVOID A COLLISION with others on the mountain. I can't stress that enough, and will say that MOST of the straightliners I see are grossly out of control and are serious hazard to other people. I and others get slammed by these jagoffs every year...skiiers and boarders alike.
Years ago, I was in a closed course, Super G race on a black diamond run in glare ice conditions. The 8 or so gates were spaced so wide apart, that you could take a beeline to the bottom if you lined up right. We all talked about how the winner would be the person with the biggest stones to flatbase all the way down, without feathering an edge. We all lamented over the ominous fact...that someone WAS going to get hurt. Sure 'nuff, one of the girls got slammed and hauled off unconscious by the ski patrol. We did the math, based on run length and time, and estimated speeds around 55 mph. It was "interesting" under controlled conditions, but definitely NOT something to do on a run opened to the public.