How to politely refuse the boss's dinner?

The most important thing to politely refuse the boss's dinner is to have a reasonable reason to make the boss think that you want to go, but you can't go for other reasons. This is the best and the most euphemistic.

You can find an excuse, preferably one that sounds real, such as things at home. It's best not to refuse the boss's dinner directly, or he will think that you don't give him face. And there is no way to refuse every time, otherwise he will know that you don't want to go, and may even be unhappy because of your previous behavior of making excuses.