Don't pay an invoice until you know you have got what you've paid for. Ok this is pretty obvious if you're in a retail shop buying a printer, but it can be a bit more challenging in terms of things that get delivered to you.Be particularly wary if your supplier asks for part-payment for a product or service but won't clearly articulate this on an invoice. Make sure that, if you are going to pay before you get something, that the invoice clearly stipulates the exact details of the product/service and include Terms and Conditions for a refund, should the product or service not meet your expectations.
Its a lot harder to get back money you've paid than it is to not pay for something in the first place. Say for example that you send an order to a supplier, they send you an invoice, you pay it and then the item is shipped to you. But the item that arrives by courier is a different model to the one you bought. In these situations its very important that this is noted at the time it arrives and that you don't accept the delivery.
To ensure you don't get stuck with something you didn't want, make one person responsible for receipting goods and ensure that they check every good against the order/invoice details before they sign for it.
Make sure that, any time you have 'an incident' related to the above, that you make a note of it against the supplier (see Performance manage their suppliers).