Build relationships with people of influence by commenting on their blogs. If your comments are thoughtful, they'll want to know you as much as you want to know them.
Engage in conversation with people on Twitter and in LinkedIn groups.
When you have a specific need, get introduced to people who can help you through LinkedIn. If you don't know someone who can make the introduction, contact people directly with InMail. Just explain why being included in each other's network is mutually beneficial.
Solidify online relationships by meeting in person.