It takes a good camera with fast shutter to get good action shots. If you have one, or get one, then follow the bike as it hits the jump and click it when it reaches the point you want the picture to be at.
If you are using a point and shoot, or a cellphone camera, all you can do is practice following the bike and press the shutter well ahead of where you want the picture to be. How far depends on how long it takes your shutter to click. It all take practice regardless. Following the subject will keep the bike/rider focused, and usually blurs the background. Conversely, holding the camera steady and attempting to capture the bike/rider as it passes thru the frame results in clear background and blurred rider. You have to have an SLR and the right settings to get clear shots of both rider and background.