My method: with the blade held horizontally in a vise, I start my file cut at the end of the bevel closest to the the center and file across the bevel at a 45 degree angle, moving toward the end of the blade and maintaining the original bevel angle. It should be at least a 10" flat file with a handle on it. Use both hands - one on the handle and the other on the back of the file. Always file only in the cutting direction, lifting the file off the work on the draw stroke. Stay at the beginning spot until the file begins to get a bite, then extend the cut area toward the end of the blade. Return to the starting point over and over until the whole bevel area is 'white' and you have the edge you're looking for.
Turn the blade around in the vise and do the other end. You also may have to take a few light strokes on the flat under the blade only to remove dents in the edge. This is just a dressing, never sharpen the blade from underneath. Never try to sharpen a blade on the mower.
Using a bench grinder, just keep the work moving to avoid overheating, and try not to remove excessive material. Always balance after sharpening, particularly if power tools are used.
I have two mowers that take the same blade, and I have 5 blades. I clean, sharpen, balance, paint and store the blade every time I take one off, so I always have three prepared blades ready to go. I don't know how many years this system will last before I have to start throwing away blades, but it looks like the 5 blades will last for many years based on the wear I've seen after 3 years using this system. It basically comes down to a sharpened blade every season for each mower. Typically a blade will wear out first at the airfoils behind the cutting edge. When these get thin and start to lose their shape, the blade is finished regardless of the condition of the edges.

I've always used a file with saxman's method. I don't like using a bench grinder because it's hard to control not getting the blade too hot and that take the temper out of the metal.

Saxman, thanks for detailing your method. From your description, I'm not sure whether you push the file up the bevel or down the bevel. I've tried to link a picture of what I mean.

Here is a link that might be useful: mower blade pic

I go up toward the edge from inside to outside. But usually I grind 'em on a bench grinder because the edge is dented.

Either by Hand or with a hand or bench grinder . The two important factors is not to over heat the blade or take off more metal than is necessary . By following the original profile of the blade will give you the best results. Also 1/8" to 3/16 of bevel is usually sufficient surface regrind . After yrs of having the blade on a nail for balance check , I now use a static bubble tester which works great .

Agree with no over heating the blade. A few strokes with a file on each side is typically all it needs. Just wanted to get a sense of whether one generally moves the file up the bevel or down the bevel to achieve an edge.

Down the bevel towards the cutting edge . You don't want a scapel sharp edge , usually I give a last file pass aginst the grain to take of any small burrs . Butter knife sharp is more than adequate really !