I had a moan the other day i do a care home every month, hadnt been paid for invoice dated 1st mar,1st april and was there doing the may clean that day. manager got on it and they have paid by BACS

I know these people do pay but they need encouraging now & then.. Acceptance is the key, I havee another commercial job & my biggest they are good as gold, often pay within 2 weeks!

and definately within the month.
As DEEGE said alot of companys pay when they want anyway so as long as there is some sort of system to it, i.e you know you will be paid 2 months after invoice, once your in the swing of it its not so bad.
I used to let it stress me thinking that unless i give the job up they will always owe me a clean, but then realised if someone were to sell me this job for the value of one clean i would snatch there hand off!
I think the rule for commercial is, as long as there is some sort of rhyme or reason to there payments & you can accomodate them everything should be ok.