While physics prevents you from getting more energy out of a system than you put into it, there are ways that you can conserve the energy in a pressurized system by extracting that energy more efficiently.
One example is the double (or triple) expansion steam engine
http://en.wikipedia.org/wiki/Steam_e...ansion_engines where the exhaust from the high pressure cylinder is used to feed a lower pressure cylinder. I wouldn't recommend this for an FRC robot's pneumatic system, but it is interesting to know about from a themodynamics standpoint.
Jason
P.S. While I was looking for a good link to explain multiple expansion engines, I stumbled upon this site:
http://www.animatedengines.com/index.shtml where someone has taken a lot of time to show off how a number of different engines (including several variations of Stirling Cycle engines) work.