Dont wait to get paid
In the early days, you need to get paid quickly.
Tell your clients that your payment terms are seven days from the imvoice date, and you will be invoicing the moment your work is done. Better yet, request a 20% deposit with the balance payable within seven days.
If your cash flow is really tight. call clients the day before your invoices are due and check that your that your invoice is due to be paid on time.
Many small businesses will be able to accept short payment terms but larger companies will have preset payments terms that they cannot deviate from- so accept that not every client will pay you according to your terms.