Invoicing
It may not sound sexy but this is a critical task if you are a business that invoices clients.
Make sure you agree invoicing terms and seek staged payments if appropriate. Have a clause to charge interest after say a 30 day term.
Once you have sent an invoice, check with the client they have received it and it is payable. Don't wait until the end of the payment period to find out they don't have all the details.
Chase the client as soon as the payment date has passed. You may have good payers but you will often need to chase.