I work in a collection company. If you want more detailed help please let me know.
Firstly: you have to legally secure your payment in case your client makes problems in the future. That means a written contract that specifies what you do, when and when he pays. This should be signed by your Client, clearly in full name and company stamp.<p>In the contract you include interests for non payment (there are limitations - depends on the country). You should give him bonus for paying on time (eg. 5% off).<p>Give a short time for complaint - include a clause that if no complaint is officially filed the service is deemed accepted.<p>Ask the Client to give you a confirmation when he receives your invoice, and when your service is performed/received. Keep track of everyting you do for him and confirm in writing (e-mail is enough) that he accepts it.<p>Please NEVER trust anyone - I know small family businesses with over $100K of unpaid invoices. Trust is good, but everything changes when you realise you were screwd and did nothing to prevent it.<p>Practice shows that calling every single day is an efficient motivator. Your Client probably delays payments to others as well, when you bother him a lot you'll be the first he pays to.<p>Give deadlines and keep them. Ask him always to pay today, not tomorrow, not by the end of the week. Escalate sanctions. If you shoot all your weapons in the beginning, you will have nothing to say to him later. So first tell him that if doesnt pay today, you will remind him every day. Then tell him that he lost his bonus, then that you will charge interests, then that you will stop the service, then that you will contact a debt collection agency/lawyer.<p>Ask him to make smaller partial payments (could be use in court as a debt acknowledgement.<p>Be always mentally/financially prepared you'll not receive these payments at all. And act fast, statistics show that succes rate drops dramatically when invoices are older than 6 months.<p>Cheers!