If you own a bike, but have no suitable space outside to store it, keeping it against a wall in your home is the only real option. If you find yourself in this situation, a portable DIY bike rack is a perfect solution to keep the bike in its place.
Fuego316 shares his process on Instructables. The tutorial is pretty self-explanatory; all you'll need is about 20 feet of PVC and some L and T connectors. Just arrange them according to the instructions, and you'll be finished in no time.
Apartment PVC Bike Rack [Instructables]