Chris, I've file my Makita chain right on the machine with a chain saw file, straight accross like a Rip saw. Use a file big enough to touch the tips of the teeth and not fall in the gullet, this steepens the angle of the tooth just slightly, seems to be less grabby in hard woods. File each tooth the same number of strokes just like a chain saw.
Keep your chain sharp and well oiled (I use bar oil in a squeeze bottle every couple of mortices) and your mortiser will last. My Makita is now 14, and on its 2nd chain. This same mortiser has survived 12 years of itinerant timber framing including at least 5 Guild rendezvous.
