I do work for several small firms. I have a small firm that tends to pay slowly (I am supposed to be paid in 10 days but for them it's usually 30), but I've always been paid over several years. I do good work.

Two months ago I sent them a large invoice (about double the usual) because of doing a lot of work in a short time. After 53 days and about 4 reminders by email and phone, they finally paid me only 1/ 2 that invoice, saying they were waiting on some payments themselves. This never happened with them before.

They still owe me1/2 of that invoice plus two later invoices.

Having received a half payment a week ago, I don't even know how to start this conversation about payment with them again. At least they haven't asked me to do any more work in the meantime. They owe me several thousand.

Any ideas? Should I just give wait and give them some space, or what can I say or do or try get paid again before too much longer? Thanks for any help.
Additional Details
Thank you all for the wealth of good suggestions. I got something valuable from EVERY answer, for now or the future.

I'm not ready to sue this client yet or send them to collection. There will be plenty of time for that. I'd like to keep the client if I can..

I sat tight for a week while reading your suggestions. Yesterday the client called and asked me to do a small project. I said I could, but asked about getting paid for the prior work. The client offered to pay me in advance for the newl project and said he'll start getting the balance paid off. A check for the new project (and for some prior work) should arrive today.

I'm glad the client still needs my talents enough to find a way to pay for them. I'm going to keep asking for advance or partial payments until the balance is brought current..

I plan to raise my rates for this client in January and allow larger discounts for prompt payment, as a roundabout way of charging interest for late payment.

Thank you all.