One of the first points I always make to anyone who asks me about the ins and outs of custom Drupal development is to spec out your projects nice and tight and to get customer sign-off on those specs. Time and again I find this is the biggest source of failure for developers.
I know this because I've been there. It's an easy trap to fall into. It's easy to be eager for the job. The natural instinct is to get on with the work once the client says yes. If we don't take the extra step to define exactly what work will be done (along with the time frame and cost) we'll eventually realize that the work will never be done, the time frame will extend and extend, and the cost per hour will plummet.
And that last bit is why custom Drupal developers fail. If we tell clients our hourly rate is $xx per hour, set an overall delivered cost for a project, and don't define the deliverables; the project will be hard to gain client acceptance, forcing us to invest more time, which dilutes that hourly wage.
Don't let this happen to you. It may seem like the client has all the leverage. After all he has the checkbook. The truth is, once you're his developer you're the gatekeeper to his new and shiny site. Get buy-in up front to exactly what you'll do. Make a list, and get the client to sign-off on it before work begins. As you complete the items on the list get sign-off on each one.
Before you know it you'll have every item knocked off the list and it will be clear to you and to the client alike that your project is complete as per initial specification.
I don't know of a better way to keep the client focused on the job as it was initially defined. I don't know of a better way to help a client see that creeping elegance is OK, but that it's something that comes at additional cost.
