Just a tip or two. Keep your files clean with a file card or small wire brush, keep it dry, and never, ever put oil on a file. Oil will prevent the file from working properly and you could end up with scuffed or cut fingers, and a poor filing job too. If you don't have any solvent to remove the oil, a little alcohol on a rag works well.

Thanks for the tips!

Machine sharpening of the blade seems to harden the metal of the blade to the point that I can never seem to be able to truely sharpen it again with a file.
Extra Tip: If you can't seem to keep your chain tight, try flipping your bar over. You should really rotate this regularly to even out wear.

That would explain a lot --- it does seem like sometimes sharpening a chain with a file does little to nothing. I'd just assumed that was when the chain was really worn out.

On the other hand, Mark thinks that some machine sharpening might be better on the chain than others --- the most recent time we got our chains sharpened, they were still very sharpenable with the file afterwards.