Dwolla is something you might consider looking into. Squarecash is also great, but I believe there are limits on how much you can send/receive in a month, so if you do large scale RE, it might not work.
Anyway, I think this is a business where you have to be polite and courteous and willing to listen to tenants problems, as longs as they're holding up their side of the bargain. That means that if they call you and give you a litany of reasons why they can't pay rent on time, you ignore pretty much all of it and say, "Give me a date that I can expect it from you with the late fee. If I don't have it in my hands on that date, I'll have to call my attorney."
I'm willing to be friendly and chat with good tenants and all that, but this is a business relationship, so the moment you start to get into their personal struggles and make exceptions for them, there's a problem.