I bought one of these sharpeners from the Grace Company a while back and I love it.
http://truecut.graceframe.com/cart/s...sharpener.html
They supposedly sharpen virtually any brand, any size blade without having to remove the blade from the cutter. I have several different size cutters, three different brands and so far it works on all of them. It won't make a blade last forever, but has really helped to extend their life! I just run the blade through the sharpener several times after each project, or any time it feels like it needs it. I only paid $24.99 for mine. If you shop around, I'm sure you can find it for less than the $39.99 listed at the site above.
There is also a tutorial that shows how to use it here:
http://www.youtube.com/watch?v=75VDKOu-kqE