Recently I played my acoustic guitar after a couple months of only playing my classical guitar. My fingers have softened considerably compared to when I was playing the acoustic regularly, and it was painful for the tips of my fingers on my left (chording) hand.
Is there anything I can do to permanently toughen up my fingers? If not, is there any other way to keep them tough besides playing regularly? Are there things I should avoid doing that can weaken or damage my calluses?
I'm particularly interested in first-hand experience; if you propose a method, how did it help you and what was the timeframe of your results?

fingers,hand(existing),physical-aspects, something else? Help me out here. – Matthew Read♦ Apr 29 '11 at 15:30conditioningcomes to mind, but hrmm. – Rebecca Chernoff Apr 29 '11 at 16:06body? There definitely should be some tag about the physical conditioning and maintenance aspect of being a musician... perhaps I'll take this to meta. – NReilingh♦ Apr 29 '11 at 16:27