Ask for a late fee

Instead of encouraging early payments with a discount, use a late fee for creating an incentive. Clients who intend to pay on time won't mind signing a contract with a late fee.

How to include late payments in your contract:

• The amount should not exceed 1.5% of the project fee unless you can justify it.
• Set a due date on your invoice.
• If a client does not pay, send an invoice reflecting the change in the final sum.