Ensure that the trunk is clean and dry. Often the rubber seal on older vehicles can become hard or dry rotted, allowing moisture to accumulate.
To prevent rust in the engine area, use a lubricant spray such as WD40 to coat all exposed metal surfaces.
If you have a convertible top, leave it up with the windows and vents closed. A convertible top can develop creases when folded for long periods of time.
You should give the car a thorough cleaning inside and out, and make sure it is completely dry before storing.
If being stored for longer than 6 months, to help ensure the long life of mechanical parts as well as the tires, periodically visit the stored vehicle and take it for a drive.
Use the proper coolant/antifreeze in the engine cooling system. Using water is not recommended and can cause rust or build-up.
Be sure to top off all fluids and fill the gas tank. This will prevent condensation from forming in the air above the fluid surface, and will reduce the chance of old gas turning to varnish. There are also gasoline additives you can purchase to help with this problem.
The engine oil should be changed before storing for long periods. Old oil may have accumulated moisture, acids, shavings etc., which may deteriorate the engine parts. You may want to apply an upper-engine lubricant to keep the pistons and rings from seizing.
Over-inflating the tires will prevent flat spots from forming.
Finally, you should disconnect the battery once the vehicle is stored. This will prevent corrosion of the electrical terminals and cables.
And finally, note dates of registration, tags, certificates and insurance, etc. and plan for renewal if they expire during your storage term.