From a question I answered previously (How to find mentors and advisors for your startup?):
1. Make a list of all the top influencers in your niche.
2. Hustle to make initial contact with them. LinkedIn is a good place to get intros. AngelList, however, isn't the right venue to make cold connections.
3. After making initial contact, offer to buy them a cup of coffee or schedule a short meeting to get their advice. Persistence pays.
4. Out of all the influencers you meet, narrow down to a few who would be a good fit.
From an interview with Peldi (founder of Balsamiq):
The number one piece of advice I can give is to “do the time”.
Let me explain: most of my advisers are former colleagues whom I worked with in my 7-year career at Macromedia and Adobe. The others are friends we know from living in San Francisco, where everyone is in tech.
They are all people I looked up to in one way or another, people I did great work with in the past and that I know I would have missed once “going solo”. So it’s partially a way to stay in touch with old friends, but very smart and knowledgeable friends!
Recruiting them was easy, I just sent this email.
We don’t have any formal agreement nor do we meet regularly. Mostly I email them whenever I have a question I know they’ll be able to the answer, to or we meet on Skype once in a while (we try to shoot for once a month but somehow haven’t been able to keep a regular schedule with anyone. Things get in the way.) We all got together for a big crab-dinner feast in San Francisco in May, something I hope to turn into a yearly tradition.