Rob, any idea how much current capacity the device needs to have? Pots and rheostats are definitely an option, and cheap too, but not quite as efficient as say a DC switching regulator. As Brain said, both pots and rheostats are a type of variable resistor, and because of that they reduce the outout voltage and waste it in the form of heat. A switching regulator, on the other hand is much more efficient. Sometimes up to 80 or 95% efficient. Energy is not wasted in the form of heat as much as a resistor.
Here is a 3 amp switching regulator. This one can step up or step down the input voltage.
http://www.dimensionengineering.com/anyvolt3.htm
Here is a 2 amp switching regulator, step down only.
http://www.dimensionengineering.com/DE-SWADJ3.htm
Finally a 1 amp.
http://www.dimensionengineering.com/DE-SWADJ.htm
Depending on the load you expect the regulator to drive, these may or may not work for you at all. 3 amps is not going to run a 540 motor under a load, but 3 amps could run some other small devices.
For some more serious switching regulators, Castle Creations has external BEC switching regulators.
20 amps.
http://www3.towerhobbies.com/cgi-bin...&I=LXYCA6&P=ML
10 amps.
http://www3.towerhobbies.com/cgi-bin...&I=LXSWL3&P=ML
I am sure if you searched the web, you could find more switching regulators, but you would need one regulator for each desired voltage drop. In your example, you would need two regulators. One for the 9.6 volts output and one for the 7.4 volt output. Hope this helps.