I like the Fluker's type of heat pads because they don't stick, making them easy to move. I usually just use some electrical tape to fix them in place, and use the "feet" that come with them to make sure the cages are elevated up to allow for some air flow.

As long as there is air flow between the pad and the surface the tank is sitting on (so the pad is touching the glass tank, but not the table/counter/dresser/etc that it is resting on), you will not have problems with cracked glass or ruined furniture. Resting the pad directly on the surface the tank is sitting on, and just putting the tank above it, can result in problems such as cracked glass or furniture with discoloration due to heat.

-Jen