Checklist for Success

Gather your ideas, no matter how specific or vague they are. Visit model homes and home centers for ideas. Your ideas can be vague ("We want more storage space") or specific ("We want electrical outlets every six feet.")
Interview 2-3 architects or design-build firms. Can you communicate well? Are you on the same wavelength? Trust your instincts.
Before selecting a firm, check references and licenses. Look at similar projects designed by the firm.
The better your input, the better the final design. As you work through the design process, be as specific as possible. Ask lots of questions.
Repeat steps 2 and 3 for a contractor. Don't sign a contract until you've checked references and licenses.
Thoroughly read the contract before signing it. If there's anything you don't understand, spending a few dollars on an attorney to review it is cheap insurance. If there's anything you don't like, negotiate it. Just because a contract is preprinted doesn't mean it can't be changed.
When the architect (or design-build firm) has produced a design you like, solicit construction bids (if using an architect). Solicit at least three bids.
Stay in contact with the contractor throughout construction, even if it's just a five-minute phone conversation at the end of every day.
Don't sign off on the project or make the final payment until everything is complete to your satisfaction. The response to, "Don't worry, I'll come back and fix that" should be, "Don't worry, I'll make my final payment after you do."