try insulating the outside back and sides of the tank with some cardboard, and use at least an 8 inch reflector lamp. The reflector lamps are important for reflecting heat, and the cardboard will help keep it from escaping. I wouldnt go anywhere higher than 40 watts, unless you have a cold house or something. Do you run the AC alot? The room I keep my frogs in has stayed consistently in the mid 80's this summer, dropping at night, while the rest of the house is air conditioned.
Perhaps you could also try a 40 watt spot bulb.