Use a thermostat OR a dummer switch you can pick up for under 10 bucks at wal-mart or home depot...it plugs into the outlet and then you plug your uth into that and push the switch up and down to lower or raise your temps...you need one or the other..they have some awesome thermostats out there aswell! (I'm sure someone has some links)

Also get a temp gun so you know the temp of the bottom of the cage...to insure it isnt to hot or to cool.