Put the fermentor in a room that has a relatively stable temp that is close to the temp you want to ferment at. If the temp is too high then you can put the pail in a water bath, put a wet towel/shirt on the pail, and even direct a fan at the wet towel/shirt.... If the temp is too low...sadly I don't know what to tell you as I haven't experienced that yet.
I put my plastic bucket fermenter outside in an old, cleaned out trashcan. Then I add water to the trashcan till it comes almost up to the level of the wort inside the bucket. Finally, I suspend an old aquarium heater into the water bath and set it for 70F. I checked it after work and every morning and it never deviated from 70 in spite of overnight air temperatures in the 40s.