If I can circle this, star it, and shout it from the rooftops: if you meet someone at a meetup/conference/random coffeehouse or you Internet-meet them in HN comments or a blog discussion or whatever follow that up with an email. Even if you don't have anything to ask for, just say "Hey, it was great to meet you. We should keep in touch." Then actually keep in touch. Then, the next time you do need something, it isn't a bolt out of the blue when you show up in their inbox. (This is, naturally, reciprocal, and you should tell people that.)

This is Every Sales Occupation Ever 101, but engineers seem to have a cultural block against doing it, as if sending an acquaintance "Hey, saw your startup just raised a round / you got a new job / you had a baby. Congratulations!" is anti-social behavior if there is ever the possibility that that relationship would result in money changing hands in the future.