I started out purchasing a standard round 7/32" file and tried sharpening by hand. It seemed to work okay, they certainly didn't get any duller. I sharpened the chain half a dozen times myself, then proceeded to bring everything into the local shop for evaluation. He pointed out that my filing was wearing down the teeth because I was pushing down while filing. Then, he brought out this PFERD CHAIN SHARP® tool and gave a quick demonstration.
The nice part about this file is it prevents you from pushing down while filing. It actually has two files on it - one the standard round file that goes inside the chain's tooth, and a flat file that files down the top of the tooth. I love it! It's much easier to use, it's quiet (no motors like an electric-powered sharpener), and I'll be able to sharpen my own chains a number of times without bringing them back to the saw shop.