We've all been there: totally fed up with a job, impatient with a demanding or incompetent boss, and ready to quit in a dramatic flash of pent-up frustration. The trouble is, leaving your job with fiery flair is going to burn a bridge, which you may need sooner than you think. The smarter move is making sure you're on solid ground with your old employer -- even as you head off toward greener pastures. It's all about quitting with class. Here's how.