That is a hard one, Timo. Once you're in the position of chasing after payment, it becomes very stressful.
And you're right, most clients don't understand how much work can go into something creative that looks simple in the delivered result. Then again, they don't need to understand those details to appreciate that you need to be paid. They're paying for results anyway. :)
One thing I do before I start working on every project is define (in a contract) exactly what kind and how many (or number of hours of) revisions will be included in the total.
That way it's clear up front. If there are any disagreements about payment later, we've both got it spelled out "on paper" to clear things up.
Beyond that, if a client refuses to pay despite having understood what was included, you shouldn't do any more work until that's rectified.
Hopefully, you've also only shown proofs with a watermark up to that point, so your client can't use anything until they've paid for it.
If you've got a client who balks at these parameters up front, then you know you're probably headed for trouble.