Start polite, escalate fast. Resend the invoice with a friendly reminder and the due date. No response, send a firmer notice with a hard deadline that references your agreement. Still nothing, name the next step, a late fee if your terms allow it or small claims for a larger amount. Prevention beats chasing though, so use a contract, invoice the day you deliver and for bigger deals take a deposit up front. General info, not legal advice.
Escalate in stages
Friendly reminder, firm notice, then real consequences. Most pay at stage two.
Reference the agreement
A notice that quotes the agreed terms and a deadline gets taken seriously.
Have a last resort
A late fee in your terms or small claims for larger sums. Know it exists.
Prevent the next one
Deposit up front, invoice on delivery, net 30 max. Stop the problem at the source.