Originally Posted by Wotname
Circular whetstone (shaped like a pencil) or circular diamond file works.
Since most serrated knives have different sizes of serration on them, a cylindrical slipstone will generally not work
. What you want is a conical slipstone, it will allow you to sharpen any size serration. Spyderco also makes a fabulous ceramic sharpener that is very simple and will sharpen any size serrations extremely efficiently, without messing with slipstones. It is a much faster method as you can sharpen the entire blade instead of one serration at a time, I love mine.