The best way to do this would be to ask: "How will you be paying?" or, if you only take cash, say "That will be <amount>, please. The customer should get the idea without being told. If they hesitate or say that they will pay later, then you can say "I'm sorry, but we require payment at this time." OR "I'm sorry, but we require payment before delivery (or service)" Specifically asking for payment is always a little awkward, so the trick is to make it as soft as possible.

Say "no money, no honey".

It all depends on the situation and what you are selling, but for the most part here in the states you would just say: "Payment is due first" "Please pay before service" "Payment is due in full before service can be given" Like i said, it just all depends on the situation and what you are selling. Hope this helps!