Only file the chainsaw tooth in one direction. Never bring the file back along the tooth, as filling backward is a quick way to destroy both your saw and file. The correct way of doing this is to bring the file out, reset and push the file through again.
Chainsaw files won't last forever; they get dull as you use them. With files being cheap, it's a good idea to buy a few boxes (3-12 files) at a time, so you'll always have a new file ready to go.