I broke down and bought one of those linear blade sharpeners, works on any size blade and almost every style of rotary cutter. Here is a link to the product page at the Grace Company, who makes it. I got a great deal for mine on Amazon. It won't get out nicks or repair a damaged blade, but it will sharpen one that's just gotten dull. Blades are expensive, I want to make mine last as long as possible!
http://www.graceframe.com/site/rotar...inea-sharpener