BigFloyd is correct. Tape it to the side of the carboy and insulate it. I control mine to within 1 degree and I can ramp it up by 1 degree if I need to. If you just put it in the air, that doesn't tell you anything and as Jipper said, the temperature of the wort can easily be 7 -10 degrees higher than the ambient air temp. I'm doing a Belgian Blonde right not and my fermentation profile was to pitch my yeast at 64 and ramp up to 68 over the course of a week. At that time I'll bump it up to 70 - 72 to finish it off. You need to get as close to the temperature of the wort if you want to do this and not the air temp (or a glass of liquid for that matter). The ideal thing would be to use a thermowell and get inside the wort, but a lot of people have done temperature comparisons and have found that the temp of the wort, with a probe on the outside and insulated is no more than a degree of difference.
Fermentation control is the one thing that has improved my brewing more that any thing else.