If negotiation is getting painful, get a lawyer. Why?

 1. Freelancing is hard enough without spending mental and emotional energy with this kind of thing. Let someone else handle the unpleasant talks.
 1. You are a business, not having legal counseling is something you can't afford.
 1. I like to think all business start with the best of intentions from both parts, but often things go wrong. You need a hired gun to make sure you will get paid no matter what and to  cover your ass (last thing you want is exposing yourself to liability).

I could not put it better than Mike Monteiro (Design Director and co-founder of [Mule Design Studio][3]) did on his [CreativeMornings][1] talk - I wish I had watched [this video][2] when I started freelancing.

In some countries you can get free or affordable advice from your union or small business bureau.

[update] 

For every relationship problem in life (professional or personal) there are always 4 alternatives:

 1. **flee**: get another job
 2. **change the other** (tricky one): make him pay for the full month
 3. **change yourself**: cut expenses?
 4. **suffer**.

Are you choosing number 4?

Another tip: if you are a group of contractors working for the same client, avoid collective negotiation - it is always easier to get a raise for yourself then for a dozen people.


  [1]: http://www.creativemornings.com
  [2]: http://shelby.tv/video/vimeo/22053820/2011-03-mike-monteiro-f-ck-you-pay-me
  [3]: http://www.muledesign.com