One mothball theory might be to take, shall we say, a Type 42 and put it in a coccoon. We keep the rest of the T42 operating thus maintaining the potential of a cadre to bring the mothballed one in to service. The fallacy is that the active one under goes modification, proper maintenance etc whereas the mod standard of the other one rapidly approaches obsolescence.
A second system that was applied to both Vulcans and Nimrods and possibly other types too, is to put a number of aircraft into reserve called IRR (IIRC) which means they are actually in the standard modification and maintenance regime and used on the flight line as normal but with the fleet flying hours predicated only against the declared active jets. The cunning plan here is you reduce your maintenance force by the proportion of aircraft put in to reserve and you don't fund the fuel, spares or weapons.
The latter is probably the most cost effective way keeping your stocks up to scratch. I don't think it would work with ships because any ship needs a crew.