Why do people recommend you?

People recommend you because they like you and trust you - the 2 fundementals of any reationships.
People like you when you build rapport with them. You find things in common and they perceive you as a 'nice' person.They see you as friendly, as a giver, as someone whose comapany they are happy to be in. Being an epathetic lister can take you a long way in the building of rapport.
Trust takes time to build. Here you need to be reliable- do what you say you will, when you say you will. Commit to your promise. Committment is doing what you say you will long after the mood has taken you.You get recommended when you and the work you do is trusted.
Over-delivering and under-promising is the key. Exceeed your clients' expectaions and people will refer you to others.
A reactive but the cheapest way of business development.
AND never forget to thank your referrers in the appropriate manner. Hand-written cards are the best way.