The main problems with leaving a car parked up are the battery going flat and the tires getting "flats".
For the battery I have these intelligent chargers. Decent ones come with fittings so it is dead easy to plug them in and out without having to open the bonnet, boot, wherever the flipping battery hides on this model etc. They are generally much better than buying a new battery. But don't get a cheap charger and use that, it'll boil your battery away and keep setting your car alarm off. I generally plug the cars in when I've finished playing as I never quite know when I will get to play next.
For the tires if I am away for any length of time I get a friend to pump them up and take them for a spin once a fortnight. For the sort of cars I have it is not difficult to find someone to do such things. Low profile tires like the Pirelli P Zeros are a right pain if you let them go completely flat, you have to pump them up go for a little way, pump them up again, go a bit further, can waste a whole morning, when a quick little squirt and a blast around the block could have saved the effort.
The next set of problems are things to do with humidity and sunlight. Stuff like mildew on leather and faded patches from only getting sunlight into part of the car.
For the humidity: for the cars that have good seals I use a bucket of something anhydrous, be careful not to spill it. You can buy "drying crystals" in some shops that do the same thing, just remember to put the whole device in a bucket as it will destroy carpets and metal if it leaks and they do like to fall over as they "grow". For the ones without good seals I use a little dehumidifier with a fan so there is circulation.
For the sunlight I would recommend parking in a garage. Car covers don't work too well outside and can cause other problems.
