if you're storing it full, keep it in a cool place making sure it does not roll. it is recommended to keep it upright, but the fiberglass bottles cannot be stored upright due to their round bottom shape.
if it is not full, it really doesn't matter if it is in a hot place because air won't expand enough to cause the burst disc to blow (you'd die before it gets hot enough for the burst disc to pop).
Do not store the cylinder completely empty, especially in a place of high humidity/moisture. it will cause pitting and general corrosion inside the tank.