The way I was taught was to start by hiring several designers to design every screen, and then only hire programmers after you're completely satisfied with the design. The way these outsourcing firms get you is by offering a cheap bid, and then charging you every time you want to make a change. So start out by knowing 100% what you want, and then once they've accepted the project don't let them ask any questions or make any suggestions until 1.0 is completely finished. (If they don't understand something they can ask for clarification, but that's it.)

Always have your fights up front, never once the project has started. That way you'll get the project on time and under budget.