First off, if you are talking about R/C servo motors, they will have to be modified to continuously rotate around.
Again, assuming R/C style servos:
If you are looking at a mini sumo, then the easiest thing to do is get a set of the splined mini sumo wheels http://www.lynxmotion.com/Product.aspx? ... egoryID=39
Most people are using the above class of wheels.
Now the other option is to take the large "cross" servo horn and screw it to the inside of the wheel. Make sure it is centered so the wheel doesn't wobble when it rotates. Well, a little wobble is okay. I know this works well since my champion mini sumo "Minimum Capacity" uses this approach. It is ugly, but it sure wins a lot. If you have smaller wheels, then you can use the smaller servo horns.
Anyways, drill a hole in the center of the wheel large enough to use a screw driver to attach and remove the servo horn from the servo. Once the servo horn is screwed to the wheel, it doesn't ever need to be removed from the wheel if you can attach the entire wheel assembly using the hole in the center of the wheel to attach the wheel to the servo.
Hope this helps.