I'd like to add that even if you at a good company, but are not motivated to perform whilst at work, it is both your interests and those of the company to move on. They might feel sad to see you go, but ultimately everybody, even the boss is replaceable and life will still go on without them.

If anything, the better companies out there would even throw leaving parties for certain great employees, there would be no bitterness at all. Just thanks for their part in getting the company to where it is today.

In fact, I've heard some stories of some rather honest bosses from smaller companies (not in the ESL field, obviously) even telling promising young employees that they are wasting their life working for them and that they should challenge themselves a little more. By that, challenge themselves elsewhere.

Sometimes a employer and employee don't turn out to be a perfect fit, but one doesn't need to wait until things deteriorate and the employee gets fired. That's why people can and will resign from jobs, from time to time. No matter how hard we tend to work, we aren't all complete masochists...

Some Chinese employers are stuck in the past and believe that an employment contract equals ownership of the employee. Just remember: You are not their slaves.