I wouldn't consider 30 days without driving a long period either. I have also had unplanned situations where 2 months have gone by and no one has driven one of the Spiders. I don't worry about this any more. If I plan on not driving the cars for a while, then I do mix in the gasoline stabilizer, and pump the tires above what I normally run. As far as the fluids, I make sure that the Spiders are always well maintained, so there is really no need to do anything special. I also try not to leave the fuel tanks low, just as a matter of habit. Things do vary according to climate, and of course you do have to watch out for rubber drying out, and fluids deteriorating when a car sits, but certainly driving once every 30 days is sufficient. As Joe pointed out, driving really means a long drive (or real use), and not just starting the car up in the garage or simply going around the block.
Best regards,