You use a heater to keep them warm. A simple heater will be sufficient to get the job done. The one I have is a bit more complicated as it has a timer, but it works just fine for keeping an entire row of clones warm. I used the "Clone Heater" from CloneTech. It is a small heater that sits on top of a clone and provides heat to the bottom of the clone.
If you are keeping your clones in a room with a lot of humidity you can use a humidifier. Alternatively, if you are keeping them in a climate controlled room you may want to consider keeping them on a heat pad. This will keep the clones from drying out and allow you to move them around without getting them wet.