I've a couple of commercial accounts that are a pain to screw the money out of

One is a dental surgery that can take as long as 6 months to get the money from, only a small clean, a tenner a pop, but they have to be invoiced monthly, said invoice is then forwarded to their head office whereby payment eventually comes my way.
sounds bad doesn't it?
If the surgery was allowed to pay with petty cash I'd do this place for a fiver. it's basically two windows, and I've EVEN TOLD THEM SO!!
Their head office are barmy in my opinion, but I don't care as I barely even notice the time it takes and I treat it like a little savings account.
But I do have larger ones that are more of a worry, one of which I have to keep pushing as the guy tries to hold on to his money as long as he can.
But as I invoice him via email I also nag him for payment that way too, very gently and politely at first, but now I am sufficiently blunt that I tell him I'll be due to clean the account in a few days and I want paying before I'll do it again.
Yet another is an apartment block where I am paid by the management company...
This one is more of a worry, the residents are desperate not to lose me, they are after all paying for it out of their service charges, and they too are aware of what a numpty the the guy is...
Because I know the residents are paying whether I clean them or not I do tend to give this account a little more leeway than I should.
The last couple of times they have been due I've not done them, just put through a letter into the management's post box to keep them informed of what is owed, point out the fact I know the residents are paying whether I clean them or not, and to further point out that most of them are prepared to have me continue to clean the windows but of course that will have to be deducted from the service charge....actually that seemed to work well as I had a cheque for £240 in my hands within a few days..curious that eh?
my point is though, I don't think you can have a set template when it comes to chasing bills as each circumstance is different.
Though at least with the recession you can use that in whatever you write as justification insisting on prompt payment or the fact that you can no longer continue cleaning until payment is made in full before each subsequent clean is carried out...
Ian