I agree with Ed Sukach, in the old days I used a product called Prolong, I'm not sure what it was, but it was probably propane or butane. In Europe, and occasionally in the US you can get Tetenal's Protectan, also propane/butane. You pay 5 to 10 bucks for a very small can of the gas, Home Depot, etal, sell the Benzomatic tanks for much less. You only need to use a small amount, the heavy gas will cover the surface of the liquid. I find it greatly increases the lifetime of developers.
Yeah, I know it could be a fire hazard, but like a lot of the chemicals in the home (much less the darkroom) used reasonably it is safe. If you're a klutz, I wouldn't recommend it. Another reason not to smoke.