In a sales negotiation
It is professional, not anti-selling, to say 'we can allow 30 days to pay - does that give you any problems?'. Discuss it, don't duck it.
On an order acknowledgement
Stress your payment terms, as a condition of sale supersedes any buyer's terms. Send it to a named, responsible person.
On an account application form
Include a paragraph for the buyer to sign, agreeing to comply with your stated payment terms and conditions of sale.
On invoices and statements
Show the payment terms boldly on the front. On invoices, also show the due date; e.g. 'Payment Terms: 30 days from invoice date - payment to reach us by 14th March 201X'.
On statements, repeat the terms and indicate debts past due dates.