Strong relationships are built over time. The best time to grow your network is before you need one.
A powerful way to do that is to simply be useful. Look for ways to help others. Make introductions, offer insight, share resources. Not for payback (though it often comes), but because it's the right thing to do and builds your reputation along the way.
My wise friend Josh Linkner sums it up perfectly: “Give generously. Don’t keep score.”