I noticed your comment about scope creep and agree that it can cause major headaches expecialy if you are trying to AVOID it instead of MANAGE it.

Scope creep is a good thing. Why? Because it allows you to deliver the best value to the customer. Most people can't think of everything they need done up front and have it nicely specified in a fixed, iron clad, document. So, instead of trying to do so, build in the flexibility of managing scope by simple negotiation like this:

Client - "You know, if we added this it would really make it work much better."
You - "Really, ok, let me estimate what that will take and then you can judge what other feature you would like to bump. Unless your willing to extend the project."

granted, somewhat crude example but, if you build this kind of thinking into your project you'll find that you end up delivering better business value to the customer while communicating the impact effectivly and letting them decide.

Oh, did I mention that your customer will be much happier and pleasant to work with? Its really a win-win situation.