Seeing the evidence is sometimes enough to convince the keeper of the corporate purse to shake loose your fee. That works, but it's not legal in some places. If your contract explicitly states that the customer gets the full version only after paying, you might get away with it. Better yet, inform the customer of the unlocked version both in the contract and in a separate letter. It is best to consult a local lawyer first, however.
Late charges are another way to encourage prompt payment.
Some states don't allow late charges. Instead, you can increase your rates but give a discount for timely payment. This leads to the concomitant problem of clients paying late and taking the discount anyway. Advertising I've never found traditional advertising useful. I may be doing it wrong, but my colleagues have similar experiences.