Dave,

Depends on the motors you are using. If using the gimson brand of motors these come with screwed inserts that allow you to bolt the motor to the base. If using drill motors there are two main options available to you. Either you can use robochallenges drill mounts or take a thick piece of plastic (20mm nylon is perfect) and cut a hole in it that fits one of the profiles of the drill gearbox. You then use the large circular face plate that comes as part of the gearbox assembly to fix it into place. You are then able to bolt this to a base.

Take a look at Jamies drumroll build thread to see how this one works. I have used this method but am terrible at documenting what I do.

One of the best things you could do actually would be to get along to an event in the crowd if there is one nearby and take a look for yourself. There aren't any roboteers that would turn you away from asking questions from the edge of the pit area and show you some of the ways in which various things are fixed and attached.