Essentially there are 2 ways to get this achievement.
1. If you want it quick, you can follow the advice of "sword of life" and find a big jump and go back and forth.
2. If you intend to try and get the other achievements in this game, just play it normally and it will come in time with no real effort.
You can help things along by doing any "free-roam" riding searching for vistas, barrels etc, on a bike and just try to hit the mud slopes that edge the roads at a fair speed. When added to the jumps you'll get during races it will pop easily in time.