How do you deal with a client who takes FOREVER to pay?
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What exactly is the service you offer?
Couple ways you can effect this, Collect in advance of providing service - no money no studio time. Collect in advance of delivering product, "Record sounds great, can't wait for you to hear it. Just need $xxx to get it to you."
You are a professional, providing quality product or service, and deserve to be treated as such.