We require a 50% deposit on almost all jobs even some with accounts when a lot of materials need to be ordered.  This helps cover the cost of materials.  Usually when someone has paid half down they are a little more inclined to be chasing you to get job done and you do not have to chase them for payment.   We limit the number of accounts to those who we have done business with for years and we know they will pay as agreed.   For the few paying on account we implemented a clause in our account agreement which simply states, that after they are 1 week past the agreed upon time we reserve the right to charge a late fee everyday job is not paid for.