Files are cheap, but even a new file requires frequent cleaning and lubrication to work right. That's right, I said lubrication. Oddly enough (to me, anyway), nothing gets a file back into shape better than a cleaning with either gasoline or WD-40, followed by a thin coating of bar/chain oil. I have a small rag that I use to clean up any oil that spills out when I'm gassing and oiling the saw on my workbench, and I use that rag to wipe down the file before sharpening. There's no better way to keep a saw chain sharp, IMO, than a properly maintained file.