I love ultratherms and T-rex's Cobra Heat mats. They're the non-stick on sealed flexwatt variety, and since they're not permanently stuck onto your tank, it's easy to swap tanks, or swap to a bigger heat mat. They also give off a warmish aura instead of a thick heavy heat like those stick on types have. And if it gets too hot you can just elevate the tank off the heat mat (though thermostat is a better option if you can afford one).
-13mur 6