I've been a sub-contractor for a handfull of jobs when I first started.
I had to clean the properties at the agreed intervals, get a 'we've cleaned your property; are you happy with it', document signed by a person from the establishment; and once a month I'd send the 'documents' off to my contractor and he'd send me a cheque.
Now and again he'd need reminding; but generally was extremely good at paying.
I must admit, I did try and poach one job from my contractor; but never got it, but they are a big company and we were naive.
My advice would be, if you do find a good sub-contractor, ensure that you pay on time, even if you don't pay well; because your business is easily replaced.