I bought a rubber coated wall hook at the hardware store for about $1. My bike is hung vertically inside a closet in my kitchen.

http://www.homedepot.com/p/Everbilt-...8048/202305571

If you go this route, you might want to protect the place where the tires touch the wall. My tires have left a mark, but the wall was already scuffed and cracked as the building I live in is over 100 years old, so I haven't bothered to correct it.