Hi
I've been running my own business for a few years now and I've got a system for dealing with slow payers.
1) Find out who is the person who will authorise and send out the payment (usually some kind of account manager -- this is ideal because they have no personal stake in not paying)
2) Phone up and calmly remind them that the invoice is overdue and ask for a date that they expect that it will be resolved.
3) On the date that they give, assuming that it hasn't been paid, call the same person again and remind them that they promised that they'd give you a check for this date -- tell them that you will be in their area the next day and that you'll call in to collect a check personally.
(When they say that it can't be done for then, get them to give you a day within the next week when you can call in to collect the check.)
If they mess you about and won't commit, tell them that you are going to call in tomorrow anyway so that the matter can be resolved face to face. (Don't tell them what time)
4) Go to your wardrobe, pull out a three quarter length black leather jacket, black jeans, a clean white shirt and boots of some kind.
5) Go to their office and tell them that you're there to see the person you've been dealing with and that you're expected.
(If they say he's just gone out or busy, take a seat and say "That's OK, I'll wait -- I've got all day"
6) When you finally get to sit in front of the guy responsible, sit down and ask him to get you your check, tell him that you've got all day, so you don't mind waiting.
If he says he can't do it right now, tie him down to an exact time within the next forty-eight hours.
7) Tell him that you will be back on that day to collect the check.

If they still don't have the check the second time you turn up -- explain that he's made three personal promises to you that this was going to be sorted and he's let you down three times, so now you are going to sit there until he sorts it out.
The trick with this is to remain completely calm throughout the entire process -- if you loose your temper or start shouting they'll be within their rights to throw you out. If you are completely calm and you keep on making it their problem, it'll eventually be easier to pay you than have turning up at the office every few days.
If they start shouting, do not say anything and whatever you do don't stand up -- wait for them to run out of steam and when they've finished calmly say "OK, now you've got that out of your system, how would you like to resolve this?"
The "How would you like to resolve this?" question is the key to sorting this out. It's the question you ask at every step.
I've only ever had to go to step eight once -- most people cave at step five.
The truth is that slow payers don't respond to either phone calls or to threats of legal action, but I've yet to meet the person who won't pay if you make eye contact with them and ask for them to settle the matter. Simply because nobody wants creditors at their door.
If you get someone who won't pay even after you've done this, you don't threaten them with the small claims court, you get papers served on them.
Pity I'm so far away or I'd go do it for you -- I enjoy it, simply because people like that take pleasure in dicking people about, but generally fold like cheap deck chairs when a tattooed guy in a black leather jacket turns up and talks quietly to them.