as above really, could leave the windows open a bit to allow the air to move round, some old wheels on it, leave the handbrake off to avoid it seizing.

Maybe take the battery off so that it doesn't drain and then set the alarm off.

Also, be careful as often these lock ups are easy pickings for people wanting to take the car or bits so try and make sure the car is secure and difficult for someone to move. Check it a couple of times a week to make sure someone hasn't been in and be aware of who is watching when you do open it up or ge tthe car out.