Prevent Firing Or Leave Gracefully

A poor performance evaluation, especially after many years of good ones, can mean that your company is setting you up for termination. If possible, stand up for yourself and get the poor evaluation off your record. Write a letter contesting any claims you feel are unfair. Substantiate your position with facts and figures. Ask for a meeting with your manager, and discuss the contested items line by line, but don't be hostile.
If you decide to leave, or if you are forced to leave, negotiate a fair severance package and get it in writing. If you do leave the corporation, leave on good terms. Don't burn bridges.
The few hours surrounding your departure could be the most important moments of your career, because they help define your reputation. Your behavior will be carefully watched, and you'll be remembered for years, either as a respected professional or as a troublemaker.
Keep in mind, you want this employer to serve as a reference in the future, and your next job offer(s) could hinge on what they say. You may be one of two finalists for your ideal job. If you leave "kicking and screaming," reference-checking could ruin your candidacy.
Future employers often check references as far back as ten years. Don't lose a valuable job offer because you once left a company on bad terms.