Along the same theme on Contractors, this is my key as well. Choose contractors based on their reputation, and develop a good collaborative relationship with them. Investors who try to beat contractors down on margin versus finding agreement on fair pricing for the services to be rendered will find themselves creating problems.
The other key point on this is to use contracts and don’t give too much money up front. If you use a reputable contractor, with a contract, payment should be less of an issue.
I work a lot with contractors, and consider them part of my team, and a contributor to my success. Handle this poorly, and they will be a contributor to your failure.