After being stung once I ask for full payment for any materials specific to the job (boilers etc) up front. For a larger project like a full bathroom installation or re-plumbing a house, I usually split payments for labour into first/second fix. All of this is covered in a signed contract (if the job is worth more than £1,000 inc. VAT) for both mine and the customers protection. Not had a single non-payer since I started working this way and customers actually like having a contract. If you use a template it doesn't take long to do either.