Most of the providers ask for at least 2 references and i am not sure if the previous providers remeber me or not may be they forget after a month or so. I would suggest just as we write review for providers, the providers should rate or write a review so that everytime a new provider need not call the previous provider she can just have a look at you review and can give a date.
Keep links of your own reviews, email it to yourself and make a folder and send it out when needed.
Stick around for a while and you will realize how realistic that suggestion is.
Build up a reputation with newbie friendly providers that are willing to be references. And, yes, a P411 account will go a long way to proving your reputation.
I like htowners suggestion. I had p411 but let it lapse but so far I haven't had any problems with screening. I think all I have to say is I have the bunny suit avatar and I'm in lmao