I prefer to talk to potential clients in person or on the phone. Email just isn't a good way to communicate when you both need to really understand the project requirements.
I have my potential clients fill out a needs analysis form, either a PDF or on my website, that helps determine their needs. Then we'll develop a proposal (free or at a cost depending on the size of the project) and send it to them. We'll try to get together on the phone and go over the proposal since we can't give it in person.
When it comes to closing the sale, I have them fax or mail the signed contract back and send a deposit check before we begin work.