In any form of business it is neccessary to have a contract, you should also be aware of the "Battle of the forms".
You should also complete a detailed survey explaining exactly what work will be carried out, as well as the correct insurance, health & safety etc.etc.
As part of the above process, you should also obtain a credit reference check on any new customer, and if they are a regular customer, perform these checks at least twice a year, or when their buying habits change suddenly.
Regards,
Rob