I agree with Errand, I had someone not pay for five month, wasnt a massive amount and I could cope without it. I sent letters and emails and went in a few times. Finally last week I went in and just said I wasn't leaving, turns out the company had changed hands, two minutes later a cheque appeared in my hand and then they asked me to quote another job for them. I did some carpet cleaning for them that afternoon and before I started I made sure I had the £150 cash in my hand.
Sometimes heavy tactics are not necessary, especially if you want to keep the customer. Just politely say when will I be receiving payment for the work I did x amount of weeks ago, and hand them a duplicate invoice. Make sure you find out when the payment will be made and not just 'we will get it done ASAP'
Good luck