Nobody has mentioned the radiator or brake fluid. I left a car for two years while working overseas and the result was the heater valve corroding away due to old antifreeze becoming acidic and also the aluminium brake cylinders all got a white-ish stuff growing on the outside from reaction with brake fluid. I ended up resleeving them all (new at the time) some years later as they were quite pitted, I think if I'd been more considerate during the lay up time they'd have done better.
Also, all the tyres became permanently out of round from sitting on concrete and I put up with it for a number of years then replaced them.
If I were to lay up a car again, I'd:
--Shoot some cavity wax over any surface rust or other areas that I "probably should" get around to doing something about (it provides a barrier to moisture and is easy to wash off with kero later if required)
--Drain, flush and refill the radiator with correct antifreeze/corrosion solution if not done in the last few years
--Syphon and refill the brake/clutch reservoir(s) then bleed the entire system if not done in the last few years
--Lift all the wheel cylinder rubber boots and apply a wipe of rubber grease to the pistons if not done in the last few years
--Give the battery to a friend for his car, I have a love/hate relationship with batteries.
--Lift the car onto axle stands, if nothing else it makes it considerably harder to steal.
--Polish all the brightwork with a good brand metal polish
--Polish up the paintwork and cover the car with a soft breathable cover
But the thing is I'm lazy and probably won't actually do any of that...
Andy.
PS I saw these
Dri Plugs which replace the spark plugs and absorb any moisture in the engine. Don't think I'd bother with them though.