If you don't already know, continuous rotation servos are servos who's output shafts rotate freely and continuously rather than through a fixed range like a normal RC servo. They're great for all sorts of robotics projects and are pretty easy to make from your typical cheap RC servo with a few modifications. I picked up a few TGY-S4505B servos from Hobby King (which seem to be good servos for the price) for a project I'm working on and modified a few for continuous rotation so I thought I'd share some pics and an overview of the two ways to go about tricking the servo controller into thinking the servo is always centered.
Here's a couple pictures of the TGY-S4505B servo disassembled:
First off, you'll need to eliminate anything that mechanically limits the output shaft from turning all the way around. This means removing the tab that limits the motion of the servo on the output gear, which I filed off (compare with the first photo above where the tab was still intact):
Next you'll need to fool the controller into thinking the servo is centered all the time. You can do this one of two ways, by centering the feedback potentiometer and locking it in place somehow (glue most likely) or by replacing the potentiometer with 2 resistors of equal value. The second method requires a little more work on your part, but you can always fall back to substituting resistors if locking the potentiometer didn't work.
The stock potentiometer is a 5k ohm but you should be able to substitute in any reasonably sized (2k-10k or greater), equal value resistors for the potentiometer as shown here:
All that's left to do now is to reassemble and verify that everything rotates freely and the output shaft doesn't move if you attempt to center the servo with whatever controller you happen to be using.