Motionsports

At the start of the run, crouch slightly to begin your speed increase. As you enter the final third of the runway, (may take a few practices to nail this), crouch lower to activate your speed boost (indicated by a sound effect and motion blur on screen).

If you've timed it right your speed boost should end just as you begin your jump meaning no extra power is wasted in the air, or no speed is lost as the boost ends prematurely.

Match the pose which is shown on screen as closely as possible (likely a superman-esque pose indicating you're on target for a massive jump) and be sure to keep your pose lined up until you have landed.

1) Start by selecting 4 players at menu, kinect will always recognize you if you're playing alone and this will save you time from reloading. Using this method, you should get it in one session.
2) Begin your boost between the 2nd and 3rd "Motionsports" fence banner (I always referenced the right side as it was easier for me to see that way).
3) Make sure you jump before the first red line and you should know you jumped in time if it says "perfect" or "nice".
4) Even with a "nice" jump, as long as you maintain the pose on the screen (it's the big holographic pose taking up your top left part of the screen), you should get the achievement. At the least, you need to land 239m to get it.

Happy hunting :)