I used to work in credit control and I'd send a couple of e-mail reminders, the first being generic. These prompts used to get a lot of accounts settled (some people really do forget or misplace the invoice).
Failing that, they'd get a personal letter with an invoice copy, sent recorded delivery (that way if it went to court we could dismiss the old, "we didn't get it!" excuse). I wouldn't put the letter through the franking machine or use a window. I'd hand write the address and use a plain white envelope. I'd want them to sign for it. Sometimes I'd put a little pen'd smilie face on the outside so it'd seem super personal. I mean, it WAS personal.
From then on in, I'd do a call - even at this stage, there can be reasonable grounds that someone hasn't paid.
NB: always keep rigorous notes on what has been said, dates, copies of recorded delivery slips.
Still not paid? Court. No fuzzling about. Get an agent to do it. You can get the costs back.
I've sued big companies for £50k+ and they never even turned up in court to dispute it. That is really sad because they pay the invoices + the court fees + our fees. What a waste.
When chasing for payment, always stay polite and reasonable, even if it is your own money. You may have to read that audit trail out in court