- Activity groups. Meetup.com, Couchsurfing, Nightclasses. Charities and volunteering.
- Become a regular at a bar or a cafe. Go there frequently, even if on your own to read a paper at the weekend or one evening. Get to know the staff, as that provides a bridge to knowing other regulars.
- When you do go out for any reason, be open to talking to people. Get into the habit of it, and if you encounter good conversations be sure to get the person's twitter/facebook/phone/email, just look for an excuse to send them something that you're both interested in. Then if you get a good chat going, look for an opportunity to meet up.
- Explore the friendship networks of the friends you already have. When you go to events with existing friends, make sure you get to know the people they bring along. There's a much higher chance of liking a friend-of-a-friend rather than a random stranger.
- Send social signals about the kind of person you are. e.g. wearing a SpaceX T-Shirt will definitely attract other space geeks for chat in random bars/cafes.
- More philosophically: remember that whilst you might not meet good friends if you go to the cafe this Saturday, you certainly won't meet them if you don't socialise at all. And it takes time - for every night or day you socialise, you probably won't meet anyone interesting about 80% of the time.