Just like anything else, you have to guess where the greatest value will be, and focus your work there. Yeah, probably most people you meet at conferences will forget you--that's ok. Pick the few that seemed most promising, or with whom you got along with, and follow up with them. Don't let them forget you.

> co-authors might remember me when they're starting a new research project

If you're co-author with someone, that is a pretty good start! The work of networking is to take the "might" out of the equation. If you stay in touch with them regularly, they will definitely remember you.