Word of mouth advertising is often the most effective way. If your friend suggest a service you will most likely end up choosing that service, or at least you will consider it. You can make your customers do word of mount advertising by providing good customer service, ensuring your customers are getting what they have paid for and keeping them satisfied in overall.
Another good way is targeted advertising, for example Adwords or advertising on websites related to your business. You should never buy PPC ads on sites which pay users to click them (PTC websites). Those ads are never targeted and the conversion rate is very low.