What's the best way to promote a new service?

Consumers only know what they see. The best product will lose out to the best marketing, time after time. The best marketing comes from a good value proposition.
Good value propositions are built on understand of what specific, measurable and exact features your service provides. Find out what benefits arise from those features and what implications they have. Benefits are what sell your product. Rate your features on importance to consumers and differences to competitors, and you will find the best marketing strategy for you.
Perceived value is key. When offering a service, make sure that it provides something that is perceived as valuable by your customers. If you can, provide information that will give your customers "insider knowledge". Find out information about your customer's spend, and explain how your service will save them money. If possible offer your service for free but remind your customers of the value your product provides.
Try using a Net Promoter Score. Ask your customers "Out of 10, how likely are you to recommend us to somebody you know?". People who give you a 9 or 10 out of 10 are the ones to approach for a review or recommendation, and those who give you a score of less than 6 should be contacted to discover why they gave such a low score.
The reason that most people don't provide a recommendation or referral is simply because they are not asked to. Ask the right people for reviews, and they will provide them.