Here's my m.o.:
Step 1. Meet new people Step 2. Keep in touch with people over time (and repeat)
The first is accomplished by any method that will yield you new contacts (real-life or online) e.g. attending a startup / technical conferences, YC SUS, Twitter, HN, ..
The second is the "I wonder what's new with that guy", and it can be passive (looking at Facebook or LinkedIn activity streams then hitting them up), or active (actually asking the guy what's up via email, @mention, etc.)