I'm just getting into robotics, but have been doing all sorts of electronics for a while.
I found the $50 robot tutorial here and am planning on using that to start with.
I got the HS-311 servos the tutorial recommends (plus they're cheapest) and modified them for continuous rotation. I also took a series of photographs for each step, and wrote a small tutorial for this specific servo, since the SoR tutorial is more general and some steps had to be modified slightly for the HS-311.
I cut off the potentiometer tab and glue it down, which is the quickest and easiest solution. The servos are so cheap that it's really not worth trying to make the modification reversible, and this method's therefore the best in my opinion.
The gear that connects to the servo is all in one piece and therefore you can't just pull the connecting part out to stop it turning the potentiometer - you have to actually cut the pot.
You can see the images and read the tutorial on Flickr:http://flickr.com/photos/randomskk/2569969633/in/set-72157605559564231/
Use the thumbnails to the right to go to the next image.
Let me know what you think!