Sometimes, your current customers will advertise for you, especially if they are happy with the service provided. Most do this by telling friends and recommending the service. These referrals can be very valuable as the current customer will 'pre-sell' their friends on your services.
This type of advertising can be really useful when promoting your SEOClerks services as it relies upon one happy customer telling another.
Please tell me if you are using this type of promotion now.