Avoid doing favours
People will befriend you if they think you can be useful. Lots of friendly smiles and compliments before asking you to cat sit, loan money whatever. As soon as you stop the friendly smiles will end. When you ask a favour, they are suddenly busy. The only way is to have friends who you are of no use to, apart from pure friendship. I'm a cynical old bastard, I know, but this has been my experience.