If you're brand new to the freelance world and feeling lost, never fear. There's a great starting point called The Freelancers Union. They've been around since 1995 and they have it on lock. Membership is free. The only fees to speak of happen if you decide to enroll in one of their group insurance products.
They also have a Rate and Review Clients section. In this case it makes sense that some of the commenters are wishing to remain anonymous. It's a shame some of the comments are a few years old. The same place might be better or worse.
In Sweden, you can actually start charging interest if clients miss their payment deadline. I'd love to see that implemented here in the States. No doubt there'd be a lot less dragging of feet.