To have a friend, you must be a friend.
And to have a robust natural network filled with friends, you've got to be a thoughtful friend. This means remembering people's birthdays and other important occasions. It means giving kudos for their accomplishments. It means sending thank-you notes for referrals or favors you receive.
It also means staying in regular contact through emails, phone calls, and get-togethers. Each week, set a goal to send one friendly unsolicited email to someone who's in your network (or someone you'd like to be in your network). Also, make at least one personal phone call a week to another individual. Finally, at least once a month, arrange to meet someone in your natural network for lunch, cocktails, or just coffee.
If none of this comes naturally to you, make it a priority by putting each obligation on your to-do list.