Using a metal file is a low-tech solution to sharpen the blade. Fix the blade it in a vice and use a file to approximate the angle at which the blade has to be sharpened. Filing cuts only in the forward stroke, so you don't need to go back and forth. You will find that the blade's edge is nice and sharp after a couple of minutes of filing.
Another method to sharpen a lawn mower blade is to use an electric drill with a blade sharpening attachment. The sharpening attachment is an abrasive wheel with the right angles to sharpen the blade it also has a plastic shield to keep it in place. Once the blade is held tightly in vice, you need to place the blade sharpening attachment on the blade's edge and turn the electric drill on. The process takes just a few minutes to completely sharpen the blade. more
