Most people ask me what is the most important aspect of the event planning business. I always reply "It is the same as with any business- Networking!"
It is true that most of my business comes from referrals, but those referrals are in large part due to business relationships that I have formed over the years. Word of mouth is truly indeed the best form of advertisement and nothing helps more when that "advertiser" has a lasting relationship with you! Businesses feel comfortable working with other businesses that they have developed working relationships with. They know that it is mutually rewarding and beneficial to the livelihood of the company.
I personally find that it is easier to work with those that I have worked with in the past. You know how they operate, what to expect of their service, and you often develop a "program" of how the services are executed together. This is not to say that you only use the same vendor for every event and business deal. I have multiple vendors within a specific category (i.e. caterers, florist, publishers) because not all are a perfect match for every event.
Networking for me is really about helping others. You are not only helping other businesses, but also helping clients by introducing them to people and/or ideas that they may have not been exposed to. It is really biblical. If you bless others, you will in turn be blessed!