You can probably see that today's image was shot on a bright sunny day. It's a little tough to get a flash shot using Auto FP High Speed Sync on a bright day without having a bank of flashes. Joe McNally did a great video of a shoot he did in Dubai a couple years ago and if you count the number of speedlites you'll see it takes a lot of power to overpower the sun. Same holds true for today's image. To get the motion lines, in camera, you'd need to set the camera to rear curtain sync and be able to effect the exposure with the flash rather than the sun. It's just easier to do it in post production if you don't have access to a dozen or so speedlites. One of the big problems with using Motion Blur in Adobe Photoshop CS5 is getting lines in front of the subject as well as behind the subject. It's not a big trick to get rid of the lines before the subject, but it is a trick. To find out what the trick is, hit the "read more".
The way today's image started out was with the rider and bike silhouetted against the sky, but with the tops of the trees running along the bottom of the shot. To cleanly remove the trees, a loose Layer Mask was made around the bike and rider. CS5's Content Aware Fill can't take information from what it can't see. Masking out the object that might provide contamination leaves only the parts of the image with acceptable material for fill. As soon as the fill is complete, the Layer Mask can be tossed in the trash. Try this trick on any Content Aware Fill project where you get some weird stuff going on during your first attempt. Works like a charm.
Once the base image was cleaned up make a copy of the image (CTRL J). Use the Tragic Wand Tool (Magic Wand - W) to select the sky. Don't forget the areas bounded by parts of the bike. Clean up any skips with the Quick Selection Tool (also W). Once you have a good selection, right click within the selection and pick Refine Edge. Put about a 2 pixel feather on the selection. Right click on the selection again and pick Save Selection.
Convert the upper copy of the image to a Smart Object (right click and pick Convert to Smart Object). Put a fairly large Motion Blur (Filter/Blur/Motion Blur) on the Layer. Go to the Channels Panel and activate the Mask (CTRL Click on the Alpha Channel). Click on the Create New Layer Mask icon in the Layers Panel. You should wind up with a clear image of the rider with motion lines before and after the rider. Using a Brush Tool (B) with black as a the foreground color and paint out the area in front of the rider.
Select all the Layers and make the whole shooting match into a Smart Object. From there, finish the image in any way you typically would.