The best way to sell services is not to sell at all, but to offer help and advice around your expertise. Our approach has always been to listen first, then come back to our office and talk about the project with a colleague, before putting together a proposal of what we think will help.
By focussing on listening in the first instance, you can be completely objective about what a potential client needs and find out more about their business, so you've got a better chance of suggesting something that will really work. It also means you can make a proper judgement about whether you can really do the best possible job of helping that client.
Make yourself the expert, ask lots of questions, and the sales will follow.