Go to the thrift store, get a rusty but solid bike. This works two fold. First off you won't be crying in a few years when living outside has destroyed your bike, and if it is stolen you won't cry then either. Plus like others have said it will be easy to make it the least attractive bike on the rack. If you can; take it inside with you. Oh and by a good lock. I would consider a U lock and a cable. I prefer the U lock but it is not always doable. If possible use them together. Finally make sure you get as much of the bike as possible in the lock/locks.