Yes, you should always have a contract if you don't want to deal with the problems that come up especially when you don't have one, including, but not limited to
(lawyer talk), getting paid the full amount for your services. This is a good start, but you will want to contact a construction attorney familiar with the laws in your area to tweak these forms for your particular business and state.
http://www.amazon.com/Contractors-Legal-Kit-User-Friendly-Remodelers/dp/1928580130
http://www.amazon.com/Contractors-Legal-Kit-User-Friendly-Remodelers/dp/1928580130