Few of us will tolerate a badly dulled knife or chisel for very long before we sharpen it. Yet most of us go through life putting up with absurdly dull garden and yard tools and never give it a second thought. That's unfortunate, because most yard and garden tools are easy to sharpen, and all work much better when kept keen.
Take the hole, for example. No tool is easier to sharpen, yet most are so dull they barely function. To sharpen a hoe use a 10-inch mill bastard file, and for best results, place the hoe in a vise so you'll have the use of both hands. Hold the file at a 45-degree angle and push it away from you while sliding it sideways across the edge of the hoe so you sharpen the full width of the blade.
Use firm stokes (holding the tip of the file with your left hand will help) and lift the file off the hoe on the return stroke. If your soil is rocky, the 45-degree bevel will be dull quite quickly, so you might increase the angle to as much as 85 degrees. Keep your hoe sharp with frequent filing and you'll be surprised how nicely it chops off stringy weeds it would merely knock down when dull.
Shovels and spades are also easier to use when filed to a 45-degree angle. The sharp edge is especially helpful when digging through sod or using the spade for edging. File the bevel on the inside or upper surface of the blade, again working with the tool held in a vise, if possible. It's much easier to produce a good edge if the tool is held rock-steady and you grip the file with both hands.
Rotary mower blades can be sharpened with a file if they are in fairly good shape. But after you've run over a few rocks, the blade may be too badly dulled to file in a reasonable length of time. In that case you'll need a grinder or belt sander. I find a belt sander is faster and easier to use, but with either tool you'll have to take the blade off the mower for sharpening.
Before you do that, however, remove the spark plug so the engine cannot possibly start up. Then turn the mower over, jam a piece of wood between blade housing so the blade can't turn, and remove the nut securing the blade to its shaft. Note the position of any washers so you can replace them properly.
Then grind or belt-sand the blade to a sharp edge, maintaining the same bevel ground onto the blade at the factory. Work slowly, with light pressure to avoid overheating the blade and destroying its temper. If it takes more than a few minutes to grind away nicks and produce a new edge, you will be smart to buy a new blade; your old one is too dull to bother with, and may be out of balance as well.
Another easy tool to sharpen is the grass whip. It looks like a golf club with a serrated blade instead of a head. Most of these have a 25-degree bevel on the lower surface of the blade. You can touch up the double-edged blade with a file if it isn't too dull.
Otherwise, use a grinder or belt sander, maintaining the edge bevel ground at the factory. If you use a belt sander, choose a 120 or medium-grit belt and make sure the belt travels away from the cutting edge, not into it. Otherwise the belt could snag on the sharp edge and tear.