1991 Honda Accord My car won't stop overheating!

I have a 91 Honda Accord that won't hold coolant. I have recently changed the thermostat (the car did fine for a week after), and there is no coolant in my oil to indicate problems with my engine block. I am not finding any evidence of a leak outside of the car, but my reserve tank is draining rapidly and my car continues to overheat. Can anyone offer any ideas as to what the problem may be??