One skill that other people haven't mentioned, but is incredibly useful: keeping up with people. Even if you talk to everyone you meet, and get 5 phone numbers

One skill that other people haven't mentioned, but is incredibly useful: keeping up with people. Even if you talk to everyone you meet, and get 5 phone numbers a day, if you don't actively plan things and hang out with them, you can never become friends with them. Follow the other commenters' advice, but also make sure you consistently plan things with people.