Undersize the heater for the tank, and make a habit of touching the outside of the tank a few times daily. Undersize heater is less likely to cook the tank if it sticks on, though going through more cycles will shorten the life. I've had one heater stick on, a cheap one. The rest didn't heat when they died. I've gone through countless heaters, and keep spares on hand.
Touching the tank is a no brainer, you will feel a too hot or too cold tank.
Heater controllers;
http
/www.jehmco.com/html/temperature_controller.html I'm running a ETCI-1R on a 150 gallon tub outdoors, 1,000W bucket heater, which is cheaper than that wattage in conventional aquarium heaters. They have no built in thermostat, nothing to break. The cheap internal thermostat on heaters, which is often just a bimetallic strip with contacts, is total junk compared to a heat controller. The thing is built better than my house thermostat.
The traditional safe way is as Bignose mentioned, which is good for a larger tank, but unworkable if you have many smaller tanks in a small area as I do. Biggest problem is getting the two heaters balanced out, something you can get close, but never perfect.
Don't beat yourself up over the heater, it happens to the best of us. I lost some corys & bristlenose years back, the angels growing out didn't mind near 100F.
