I run into this in one of my companies often. The two big questions to answer are: 1) whether or not you want to have a continuing relationship with this customer or not and 2) whether there is any money there to collect. If you don't care about the relationship, then you have many options, including collection agencies. We use a small one, with good results. Their first step is to send a collection letter. If the account is paid within 30 days, no commission is payable. After that it is 30%. I recall that about 90% are settled within that 30 days. I don't know if this letter/30 days grace is normal policy or not, so you would need to inquire. We continue to do business with many of these customers. You need to consider what your time is worth too...
If you want to continue doing business, then you need to move from hounding phone calls to a more "consultative" approach. I have found that sending a letter explaining your "need" to be paid, but also your willingness to work out some terms that can satisfy both of you, works well. Charging interest, post dated cheques, credit card number... these are all viable options. One thing I learned long ago, is that the customer bounces a cheque, it is much easier to collect if it does go to court, simply because that mere act of writing the cheque means the customer acknowledges that they owe the money. If you have nothing, they can debate this fact. Small claims court is quite simple, if the amount is within their range, especially if you have a bounced cheque.
All of the above not-withstanding, if your sense is that they are going down, there isn't really much you can do but hound the crap out of them in the hopes you'll get something.
I read about a company that hired a smelly wino to camp out in the account's office until they got a cheque. Don't know if it's true or not!
If you want the name of the Collection Agency we use, PM me...
Good luck!