I use a lamp dimmer I got from Walmart for $10. I'd like to get a thermostat eventually, but for now, I am going with the lamp dimmer. Having wires running into the cage when she's a hatchling makes me nervous- I don't want her to skip out on me!!
The dimmer works, but it doesn't adjust to changing room temp. So what I did is I spent 1-2 days with a regular, old fashioned thermometer in the tank, over the UTH, and kept adjusting it by tiny amounts until it was perfect. Now I know what the "base" temp is for summer, and when winter comes, I will probably have to monitor it and "dial it down" if necessary. But at least I know it won't burn her with the 100 degree temp it sets all by its lonesome self.