Cash Flow - collecting Debtors

As my mentor Allen Pathmarajah tells me :-

Turnover is vanity

Profit is sanity

Cash flow is king and queen

Collecting outstanding debtors and then systematising invoicing and debtor processes to ensure that all customers pay in 14 days from date of invoice in order to get the cash flowing and letting you breathe easier- would be a win.

What are ways that you  collect outstanding debtors in less than 14 days?