Firstly, make sure that you get all estimates in writing along with an outline of what works are to be done, start and completion dates, security and safety, waste disposal and hours of working. For larger jobs there is an industry wide contract available in the form of JCT 2005 (Joint Contracts Tribunal).
Avoid cash deposits. In most cases these are not necessary.
Some builders will want to be paid 'cash in hand' to avoid payment of taxes. If they are prepared to act dishonestly in that respect, can they be trusted to deliver a proper service? In addition, if there are no paper records, you may not be able to prove that payments have been made in case things go wrong. An invoice proves a relationship and can be helpful if a dispute arises.
Ask for details of the required payments. For all but small jobs, the builder may ask for payments at specific stages of the work. The payments should reflect the amount of work already completed.