Speak open and friendly to your client that it is important for you being paid on time. I would always call over email a client. You get a better feel on what's

Speak open and friendly to your client that it is important for you being paid on time. I would always call over email a client. You get a better feel on what's going on. If multiple follow up attempts fail, I'd speak more serious and eventually involve a lawyer. You could also explore selling your invoices. Factoring can be a good way to mitigate the cash flow issues.