I get this question a lot so I thought I'd share:
Put your camera on manual exposure and manual focus. Make sure both are exactly where you want them. Have your subject walk, jump, skip (whatever you'd like them to do) while you take multiple shots using rapid fire mode. Here's the trick for easy photoshopping- hold the camera very still and do not follow your subject with it or else the background will be the one moving (and that isn't what we want!).
Now, when you're ready to photoshop then take out the shots you want to use. Click 'v' or (the move tool) and drag each shot on top of the final pose. Hold down shift while doing this so the images will all be aligned as they are dropped. Now click alt+add layer mask on top of each of the layers except the back one. Select your brush tool and using a low opacity paint on the mask slightly revealing the moving person. Do this on each layer until you have the effect. Compress the image and adjust as desired! (Do not adjust layers before for best effect)
my actions are also for sale as shown in the last image. click here:
Amy-Rose King Photography