There are a number of electronic temperatue heater controllers on the market. Personally I use the JBJ True Temp Digital ($90-$100 online).
Aquarium Heating & Temperature Control: JBJ True Temp Digital Heater Controller
It can handle up to 1000 watts total but only has one cord connection so I run multiple heaters off an extension cord plugged into it.
It comes with a removable temperature probe on a decent length (5') cord. It has a built in memory and does not need to be reset after a power failure.
It can be calibated to 0.1 degrees off another thermometer and has a indicator light which shows when power is being feed to heater(s). It also has a Large visual temperature display that you can check at a glance.
You basically set all your attached heater thermostats at least a couple of degrees higher than the controller activation temperature. Then when the controller activates it sends power to all the attached heaters which should come on until the controller kills the power.
Be aware that it has a one degree tolerance. This means that if you set the controller activation temperature to 78 degrees the controller will activate at exactly 77 degrees and shutoff at exactlly 79 degrees (always a two degree swing during heating cycle). It also is Only available in Fahrenheit usage (I would have prefered a Celsius readout option).
I find they work well on large tanks with multiple heaters. I mount heaters on each end of the tank and place the controller temperature probe in the back center. I no longer have to play around with the thermostats on each individual heater. I also use them on my heated water storage barrels so I can tell at a glance that the water temperatures match.