As a contractor, you'll save yourself a lot of time and headaches if you sit with a client and work through mockups prior to the beginning of a project. This does two things that are worth their weight in gold.
First, it makes sure that the client is comfortable with what they are getting. Presenting them with something they can see and clearly understand helps eliminate miscommunications. Second, it makes sure you have a clear understanding of the clients needs and makes sure you start development in the right direction.
If a prospective client doesn't have a clear vision and plan for the project before you start. Tell them that that is the first thing you will need to help them address if they bring you on for the project. It's ok to bill them for the time spent helping them build out the project plan, but make sure one is in place before starting or things will drift and both you and they will end up frustrated and burnt out.