You should think of how using your current customers attract more new clients. The word of mouth still has a strong power, but it has changed its appearance.
Use your network, then use your customers network! For this it's a good idea to put Facebook's Recomend or Like buttons in your webpage. It helps to win some audience from this social network. Or even better, you need to create a field with personalized message which could be send by your customers to their contacts' e-mail or phone. People have more trust in people they know. So let others recommend your website.
Well with SMS message you can always reach the perspective customer better, because nowadays all the people have phones somewhere near them 24/7 and the message can be read at any time, no mater if you are at work or at home or at meeting. In other hand with e-mail you can't be so sure, because it can go to spam or newer be checked.
I'm using www.smspoint.co.uk which actually can be integrated in your webpage's system.
People who find your website useful should have a possibility to announce it to their network. But let me clarify one point, your customers will not 'work' for you if you will not give them any reward for their effort. So make sure that you can offer something in return of their active participation. It can be not very expensive but useful prize.
Promotion is more effective when participants (your current clients) have an equal opportunities and are requested for specified and reachable goals, such as new client attraction.
Let your customers work for you!