You should always be on the look out for new people to join your network. Introduced to someone down the pub? Make sure you have a friendly discussion to find out who they are, what they do, and if they or anyone they know might have some needs. We're not talking a full on sales pitch, but always be curious and attempt to get to know people - you never know where it might lead.
When you place a candidate aim to give them great service so that not only do they want to use you again, but you also get referrals from them. You could even find yourself in a situation one day where you call them about a position available and they say "I'm sorry, I'm not available, but I've got a friend who would be perfect…" Similarly, you want to provide great service to your clients so they use you again and again, and also provide extra work, leads, and references if you need them.
The great thing about networks is that they keep growing. You have to remember that once you have a small group of people in your network, you are actually attached, albeit indirectly, to their networks as well. Look after the people in your network and eventually you will hear people say "I can't help you, but let me get in touch with a friend of mine who I think can…"
Informal networks have now been given a new lease of life in the digital age thanks to sites such as LinkedIn. You now have the ability online to build virtual networks and ease the flow of communication. But never forget to maintain your offline network too.