Jeffrey Zeldman, the Dali Lama of all things web writes about Getting Paid.
It's somehow reassuring to know that it doesn't get any better the more successful you are.
I've been self-employed since 1990, save an almost four year stint with a day job from late 2003 to mid 2007.
In the first block of self-employment; getting paid was nothing like how you or I treat our bills. We get a bill, it's vaguely reasonable, we queue it up and pay it. My customers got a bill and treated it like an opening salvo in a contentious dialog. The people who love your work are not the people who write the check. The people using your work are not the people who approve the invoice. The bigger the client, the more they are leveraging you for cash flow. When I was at Apple, Peter Oppenheimer (Apple CFO) explained to us how Apple has built, shipped and sold the product before they've paid the component suppliers for their goods. I.e., the vendors provided the float. I enjoyed the memories of waiting 45-60 days for payments from Apple. (Think about that. You do work for someone on November 1st, invoice at the end of the month and get paid for that work between January 15th to Feb 1).
With this kind of jacking around, I tended to turn inward - fix what I could fix. I tried to be better about setting client expectations (yes, you have to pay me before everything is done; it's a progress payment) Try to be better about specifications. Always touch base before you send an invoice. (what? you're unhappy with some part of the project and you tell me at 40 days out on the invoice when I call and ask where my money is?). While all those things improved the client experience, it didn't really help getting paid more quickly.
The second block of self-employment has been better. For the first time in my career I got hired through a job house for a nice, long project. Great work, great people and I was on payroll at a consultant's rate. I was paid the following week! Lately the smaller clients have all been in the 'pay him as soon as we get the invoice' camp. Those are great.
And I'm currently working on a BigCompany project as a sub where the client is a quick pay. Shut my mouth and slap your grandma. Who'd a thunk it. I hope they don't hire Peter Oppenheimer.