If you are storing your car for the winter, it is better to put the car up onto jack stands.  There are quite a few reasons to do this.  First, it makes it a lot easier to work under the car. Second, it helps air flow underneath the car. If you have a concrete floor moisture seeps up through the concrete so it is better to have it off the ground allowing better air flow under the car. Third, it keeps the tires off the ground so flat spots don't develop keeps the tires away from the moisture as mentioned before. When you place a second generation Trans Am on jack stands use the recommended locations for placement of the jack stands. The jack stands should be located on the frame of the car (see below).

     You should make sure that you have the car evenly supported and level.  A good way to check is by using the doors, hood, and trunk. They should open and close just as easy as if it were sitting on the tires. If you have a hard time closing and opening the doors, then you don't have it sitting right and you should take it down and do it again. (NOTE: Take care while doing this to not damage your paint.)

     Another good idea is to disconnect the negative battery cable.  Again, there are a few reasons for doing this. It saves the battery from being drained by the car (clock, etc.); it makes it easier to work on the car. If you are doing any kind of electrical work, the battery should be disconnected so why not do it now?

     Back on the subject of airflow, you also want to have your windows open a crack to allow air into the interior of the car. If it remains closed up, it can get moisture build up and result in that musty smell. Crank both of the windows down an inch or two.  

     You should also put a fuel stabilizer in so that fuel doesn’t go bad in the gas tank.

     Something to watch out for is covering your car. Be sure that you have a soft car cover that it doesn't cause scratches. Even if you do have a soft car cover, you can still get marks on your car. Not scratches, but rub marks from taking the cover off and on. You need to be very careful when removing the car cover to avoid these.  See the photos below for an example of these rub marks.