A lot of disputes with contractors start out small. They end up turning into something big because people aren't sure how to handle the issue. They'll either delay talking to the contractor (which can give a relatively minor problem time to morph into a really huge problem) or approach the contractor in a way that creates unnecessary hostility.
Assume that there's a simple solution
Never go into a dispute assuming the worst. That leads to unnecessary frustration and could close you off to possible solutions. Assume that your contractor will be able to fix whatever is wrong to your satisfaction and stay open to the possibility of alternatives.
Don't wait to discuss the issue
If you're unhappy about something, nothing good can come from waiting to discuss it. Whether the wrong faucet was used or there's a mistake in the design, you need to discuss it as soon as possible to prevent the work from continuing in the wrong direction. Everything that's done wrong has to be done again -- which delays your project and could raise costs.
Stay polite and professional
If you extend your contractor the courtesy you'd extend any other professional, it can help your conversation go more smoothly. Don't approach your contractor with a complaint when he or she is surrounded by employees -- which could be embarrassing. Ask for a private meeting to air your concerns instead.
Review your contract
Your contract is your greatest source of strength and support if your contractor proves unwilling to compromise, negotiate or otherwise fix the problem. Make sure that you know what your contract says regarding dispute resolution so that you're negotiating from a position of strength -- not uncertainty.
Write things down
Follow up on your conversation with the contractor in writing. Reiterate your understanding of the problem and the solution that was agreed upon in your verbal discussion.
Sometimes, no matter how hard you try, a dispute with a contractor just escalates -- at which point, you may have to turn to the courts for help. However, at least you'll be able to show that you did everything in your power to avoid that step -- which can only work in your favor.