If you've dumped your bike there's a good chance you've got a rough spot on your clutch, brake handle, or some other metal part. Smoothing off the roughness is a fairly simple matter if you know how to correctly use a file. It's not like filing your nails, but with a bit of practice you can learn to produce a very nice finish on your part.
When you go to purchase a file you'll find a fair number of types, such as half-round files, triangular files, round files and a number of other variations. For most things you'll do on your motorcycle an 8 to 12 inch long mill file is a good first choice. Don't go cheap or the metal will likely be soft and the teeth will dull quickly. A Craftsman file from Sears, although not bearing the lifetime warrentee of most of their tools, will serve you fairly well.
For safety, make sure your file has a handle even if the handle is sold as a separate item. The handle protects you from stabbing yourself or others with the sharp tang. It can also prevent blisters from forming on your hands, so it's well worth the additional expense. If you can't find a handle, an old golf ball stuck on the tang works as a good substitute.
Your file will be cutting metal, and you're going to be applying a bit of pressure to the part, so secure the part as much as possible before you begin. A small part can be placed in a vise, a flat part can be clamped to a table, or if the part is large enough you can sometimes sit on it to keep it secure. If you're going to work directly on your bike, just make sure you don't tip it over.
In contrast to holding a nail file, you hold a metal file with both hands. Grasp the handle with your dominate hand, then place the heel of your other hand on the end of the file and close your fingers over the forward tip. This gives you the most control and helps prevent the file from chattering (the cutting teeth bouncing across the metal rather than cutting into it). If your file is small, or you need a lighter cut, use your thumb and forefinger to hold the tip instead of the heal of your hand.
The first few cuts should be made slowly with very light pressure to remove any edges from the material that might snag the teeth of the file. Once you feel the file traveling smoothly over the metal you can apply more pressure for a fuller cut. Again, in contrast to filing your nails, the best cut will be made if you don't move the file quickly. A long, slow, and smooth stroke will remove far more metal and produce a better finish than the short fast strokes of someone filing their nails. In fact, how you use a metal file will be noticed should you do so around a machinist or a mechanic. It's a common litmus test of sorts. A good technique proves you know what you're doing around a shop.
When cutting a soft metal such as aluminum you'll find the teeth of your file will quickly clog with the removed metal. Plastics, dirt, and other debris can clog the teeth as well, reducing the efficiency of your file. A "file card " is small wire brush made specifically for cleaning a file and will quickly remove the debris and restore the usefulness. Eventually, however, the teeth of the file will dull, especially if used to cut hardened metal. At this point, it should be replaced.
To complete the repair, be sure to touch up the exposed metal with a bit of primer, paint or other protectant. In many cases once you've finished the repair it won't be noticeable at all.
Photo from Shopwork on the Farm by M.M. Jones - 1945 (it's one of my dad's old textbooks).