Single Data Access

Can someone give an example of how to implement a single data access? I'm not talking about the whole entire solution but maybe just the members. I would like to see how it would look if the service layer was not coupled to the Data Layer. I know it is not really coupled but I would like to see how it would look and how to best implement it without having a reference if that is possible. I think there is a reference now because the data layer can be swapped out by simply changing the web.config.

Thanks