For me it is rare for this to happen, but I have been there.
What I do is type up a dated letter stating the outstanding balance has not been received and is required to be paid immediately in order to continue with the service.
Then go and doorstep them.
Go to their home or business and if they are there politely explain the situation to them. 99.9% of the time you will get paid and will probably get an apology aswell.
If they are not there leave the letter.
If there is still no reply after a reasonable time period after you leave the letter repeat the process again.
Most late payers are forgetful and all they need is a little reminder.