Think of it this way your clients won t have very much to lose if you ask for all of your money at the completion of the project, because they already received the completed work. You should always send something first to the client in writing stating their delinquent status. I find it helpful to both email and snail mail overdue notices. Make sure not to use vulgarity, curse words, or threaten violence to express yourself this is illegal. Keep things professional, but DO make a huge fuss over the matter.
Email works well too.
You rely on this payment to pay your bills, to pay for medical care, to feed your children. This might put a face on the bills that have been sitting on your client s desk. Draw phone calls out to embarrass the client, making sure to bring up previous conversations that negate his or her previous comments. There is nothing more embarrassing than being called a liar. If your collection efforts have all but failed, you can try to hire out a collections agency.