A mower blade that has developed nicks, dings, and curls from hitting rocks and branches is no good to your lawn. A dull blade rips grass instead of cutting it cleanly, which leaves it more susceptible to disease. To sharpen a blade that's in fairly good condition, just clamp it in a vise every couple of weeks and run a metal file along the cutting edge, making sure to follow the angle of the factory bevel. (Always disconnect the spark-plug wire before taking off the blade or doing any mower maintenance.)
If the blade is badly damaged, it needs to be sharpened on a bench grinder or may need to be replaced. A service shop can sharpen it for you or tell you when it's ready for the trash bin. But if you have your own grinder, it's easy to do the lawn mower blade sharpening yourself. We've got some advice for how to sharpen lawn mower blades.
Begin by running the blade back and forth perpendicular to the spinning wheel to grind out nicks. This will give you a blunt but straight edge. Then hold the blade—supported by the rest plate—at the angle of the existing bevel and grind the length of the blade until you get a sharp edge.
Finally, check to make sure the blade is balanced: An off-kilter blade can damage the mower. It's easy with a plastic balancer, available at lawn equipment shops and home centers for just a few dollars. Rest the blade on the balancer; if one side dips down, then you need to grind a bit of steel from that end (not from the cutting edge) to lighten it. Once the blade is balanced, bolt it back on the mower and you'll be ready to attack the long grass.