What is a letter of demand? You can send a debtor a letter of demand to allege that they owe a debt and advise that if it is not paid within a certain period of time (usually 14 days), you will begin legal action.
If your customer is a company, you can ask the directors to provide a personal guarantee the payment of any debt owed to your business. That way if the company is wound up, you have recourse to the assets of the directors instead. Care should be taken with the wording of the guarantee, and it is best to get professional advice to ensure that the guarantee is enforceable.