Should I continue to perform freelance work for customers who keep on demanding more without paying? [closed]

Question

So a client comes to me and says it needs some work done. Basically 4 tasks, which I agreed to perform for a certain price. The customer creates the job offer (a fixed time and price job) on ODesk, I accept it, but it took some days and constant reminding for the customer to initiate a contract based on that job.

The problem is that the original task has been completed, the contract is still active, I have not been paid yet, the customer says he will pay when the project is completed, and until then, new tasks occur constantly, or changes to older tasks that require full re-doing of elements. For all of this the client promises payment. No updates from this client on ODesk, no new tasks there. I keep reminding the client about sorting out the administrative issues, but with no results. At the same time, the client pushes on continuing work, since the project is soon to be launched.

I don't know what to do.

• If I refuse to perform any work without the bureaucracy, the project will be late. But it's not my fault, is it? I am scared that I might receive a negative feedback in this case, or something even worse.
• If I drop out of this project I'm afraid I won't be getting even what I've earned.
• If I continue like this, I'll be wasting a lot of time doing stuff similar to the stuff described here, but as a programmer (yes, it has already come to similar requests, for lots of hardcoded content).

How do I communicate with such clients in such situations? How do I avoid a conflict?

P.S. The client is a small company. Different people handle different aspects of this project, and everybody introduces their own changes to the original specs.

After 2 years: Wow! Very question! Much popular!
I decided to let everyone in on the ending of this story. I've confronted the customer, explained that I will not work until I get paid for what I did, and until a hourly contract is open. They paid me the next day and opened a contract. A fruitful, but short collaboration started. Everyone was happy. Except I didn't get any feedback.