• Make sure to get a written proposal from the contractor before starting any work
• In the contractor's bid, don't let the contractor include an allowances line item for unforseen charges. 
• Establish Good Communication
• Keep track of all activities in a journal format for future reference
• If there are any changes in the work, make sure they are in writing and you approve before starting the work.
• Visually check the work each step of the way to make sure they are in line with what was agreed.
• Pay Only for Completed Work.
• Be a Good Customer.