This is happening more and more often these days.  Companies like to use you as a bank.  One way to fight this is to check them out first with a D&B report.  This will show the trend they have as a company and if you need to be leery of extending them credit.  You can always ask for payment in advance or even half down, if it's a large order.  
   Payment via credit card is another option.  Banks typically charge a small percentage for processing the charge and the majority of people know this, so you can add the 2%~5% onto the invoice.  Don't forget to tell them though that this is going to happen.  Best to have this on your quote you've sent to them.  Cover all your bases.