I use a Dremel to sharpen. I like to sharpen as soon as there's the first sign of dullness.

My neighbor was watching me cut 1 day. I hit some embedded barbed wire, causing a few sparks and swear words to fly about. He insisted that I bring the saw to his shop, where he sharpened the chain with his Harbor Freight sharpener.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93213

My saw cut just as well as my most careful and tedious Dremel job, and sharpening took about 1/2 the time. I gotta get me one!

Shrimp