There are a few different ways to convey the sense of motion in your images using different Blurring Techniques. Motion blur happens when you have a moving subject and your shutter speed is slow enough to record a trail of the motion instead of freezing it.
The shutter will be determined by the speed of your subject. Faster moving subjects will require a bit faster shutter speed to get the same amount of blur. The trial and error method is the best and pretty much only way to figure it out. Between 1/30th and 1/60th of a second should be a good place to start.
The first and most common technique involves panning the camera as your subject is passing in front of you with a slow enough shutter speed. By doing this your subject should remain sharp and everything else should be blurred. It works really well when shooting racing cars with telephoto lens. This technique may take you a few tries to get the right movement and right shutter speed but it’s totally worth it. Use this when you have plenty of opportunity to shoot the action over and over. A panning tripod head or a monopod will help you achieve better results.
Another way to use motion blur is by blurring the subject and having the rest of the scene tack sharp. For this technique a tripod is highly recommended, or at least a sturdy place to rest the camera. Set your camera to a slow enough shutter speed, compose the frame and have the athlete ride/drive/swim/roll…etc…. by you. Try to get them as they are just getting in to the frame. If your shutter speed is too slow you won’t be able to tell what the subject is and you may want to increase the speed a bit. If you can’t see the subject it means the shutter speed is way too slow.
For the third technique you will need a flash. It is very important that you set your flash to “Rear Curtain Sync” mode (meaning that the flash fires at the end of the exposure, not the beginning). With this method you create motion blur by panning your camera with the subject, zooming your lens or shaking the camera during exposure. Then the flash fires right before the end of the exposure and freeze’s the action at that point. You will get a blurred background while still retaining a somewhat sharp subject. The only area of sharpness will be where the flash hits.
There is a lot of room for experimenting with any of these techniques so go out and have fun with it!