I don't always meet my customers as they are random customers and company owners from all over the country. Luckily I haven;t been had for too much over the years, more luck than judgement, although I am chasing a couple of k ATM from someone doing a dissapearing act.Proving you did the work is technically quite difficult, but you should ideally have something in place before you start work as without a contract of works between you and the customer it can all get a bit pushy shovey in the event of any non-payment\accusations of work not done etc... We had an issue years ago where someone decided not to pay for a minor job we did and this was brought up at the time, and also another client told us without a contract of works signed before commencement of works they would not only not pay, but sue for damage to the property.
Putting contracts and forms into practice depends on your works and pricing structure, if all your work is quoted then it's fairly easy. If not then you need to quote, or work for a fixed price and price every job before starting.