There are wheels available (we sell 'em, too) that can be inserted into the bottom of your bass. They're great - I have when I have to cover long distances between car and gig and then arrive all sweaty and too worn out to play!

Installing:
You simply remove the endpin shaft (ok, sometimes it's not "simply" -- some endpins have a crossbar or other mechanism on the inside of the endpin that prevents it from falling out. In that case, you just shove the endpin into the bass, retrieve it from a f-hole, and remove or grind off whatever is preventing its removal.) and slide the wheel's shaft in. Good wheels have a flat side on the shaft that you can adjust to the right angle, that way you can use the set screw position to keep the wheel heading in the right direction.

Choosing:
It's important to choose the correct shaft size, so you'll probably have to measure the bass' endpin shaft. The most common size is 10mm (about 3/8 inch), followed by 8mm. Kays and Engelhardts with original endpins (all steel, both shaft and the receiver) are 1/2 inch; that size is typically only found on those brands, and keep in mind that some folks remove and replace those endpin mechanisms. If you don't have a caliper, micrometer, or other device to measure the shaft, there are ways to get around that. Get a piece of stiff cardstock or cardboard, and cut a notch in it. Continue to slightly widen that slot until the endpin fits perfectly - then measure the gap with a conventional ruler. Do the measurement with both inches and millimeters; as you can see, shafts are available in both metric and SAE (inches) sizes, and some of the sizes are very close.

Consider the bass and wheel you use; they are designed for use on relatively flat surfaces - but how much "give" there is in the wheel as well as the fragility of the bass still has to be considered. I use one of the less expensive wheels with my Kay because it is a laminated instrument, and while the wheel has some softness to absorb shock, the bass can take the vibration. My carved Juzek is more fragile and would call for the Gaines' pneumatic wheel, as it offers more shock absorption. Regardless, it's usually best to lift your instrument over rougher surfaces that might transmit too much shock.

Using:
I like aligning my wheel so my bass rolls sideways-- I lean the bass shoulder into my right shoulder, reach over to grab the case handle or upper bout, and roll forward. You'll get the hang of steering in no time (people usually get out of the way when they see it coming, so who cares?).