How can I protect myself against untrue written warning of unacceptable performance?

I have received a written final corrective to improve performance. I have been working very diligently to meet their expectations however the rules seem to change to ensure that I will never be able to accomplish this. I am a salesperson and I must make phone calls to contact current and potential customers. My goal in my last written review was to complete a minimum of xx outgoing calls a day. Now they documented I have not met this goal because now they are only counting phone calls made to potential customers who have not had any previous contact with our business. The warning also included accusations that are not true and examples of gossip heard from other people. I do have coworkers who have similar goals however one of my coworkers lies on reports to appear to be meeting the goals and my other coworker is not repremanded for not meeting the goals. I believe that I will be fired. I would like to know if there is anything that I can do to protect myself so that I can receive unemployment benefits when this occurs. I am looking for other employment. I am also working like a dog to meet their ever changing expectations. They seem to enjoy creating outrageous requests so they can watch me rearrange my life to try and accomplish them. My coworkers rarely work a 40 hour week but I am expected to be there at 8:00 am every day until 9:00 pm if the manager feels that it is necessary. I would appreciate any guidance you can provide.