Small shed: Available from most hardware stores for about $200-$600 these sheds are small enough to fit in any carpark alloted space. They are lockable and would be a great deterrant against potential thieves. On the downside, they are fairly flimsy, any potential thief with some tools could break into one of these sheds pretty easily. Also, they are pretty small, you could possibly get two bikes in there, but it would be a tight fit.
The Bike Box: I was put onto this Australian company after I posed the storage question in an online cycling forum. This company makes bike boxes specifically for apartment basement carparks. Having not seen one in person I can only comment on what I saw on the website. They seem to be very well made and they look very secure; a thief would struggle to get inside one of these boxes. The downsides? One word - cost. These boxes are seriously expensive, you wont get much change from $1000!
Outside on the balcony: Obviously this option is only available to those apartment dwellers who have a balcony. There are many waterproof bike covers on the market which could protect your bike from the elements, but for someone who uses their bike daily, this is not really a viable option. Taking my bike through my apartment every day and re-covering it up would get frustrating very quickly.
Inside: If you are lucky enough to have some spare room indoors you could try to keep your bikes inside. There are a few problems with this solution: 1. you could really upset others that live with you! 2. you could constantly be trudging mud and grease through the apartment, and 3. if you have many bikes, you could lose a lot of space, and for apartment dwellers, space is priceless. If you do go down this path, there are many solutions for storing the bikes, such as hooks, pully systems, bike stands and bike towers.
So which solution did I take up? Well, I have three bikes, and I'm planning to buy a fourth. There is no way I could get four bikes into a small shed or box, so putting the bikes in the spare bedroom was the only viable option. To make them as neat as possible and to try not to take up too much room, I decided to build my own four bike tower. I'd seen bike towers for sale for about $300, but I knew I could build something similar much cheaper. I ended up purchasing all the materials for about $50, and it took about 6hrs to build. It is working so far, and my wife seems to not be too upset with the situation! I have posted some photos below of the tower... it may be ugly... but it works!