Go bipolar, it works big time. Fake signs of major depression and  extreme anger If you get your salary delayed or you suspect you are about to get screwed over something, but if everything is fine between you and your employer,  act as a cool friendly laowai.

Hulk-face angry laowai.

This is where working with my partner helped immeasurably - good cop, bad cop. Normally I was the 'good' one, and my partner would be the more forceful one. When shit really was going down, we'd switch, and that would really confuse them.

Also, wildcat's advice is very, very good - be willing to compromise on small things, it's good for everyone's mysterious, too-complicated-Laowai-never-understand face. But leave them under no misapprehension that you will put up with them messing with you on salary, visa, or housing issues.

depends on the situation.

if it is a chronic problem , then get angry but with emails. they might want to talk about it in person or on the phone, but you want to create a record of what happened. remember this is business.

if it is the first time this problem has happened, then a polite email calling attention to the problem will suffice.

As others have said, it should be Chameleon Laowai. Learn to adapt to various situations and change your moods and expressions accordingly.

KNOW-WHAT-YOU-AND-I-SAID-CLEARLY-LAOWAI. I've had this problem on several occasions; I get lied to and or cheated. And then, just because I'm the foreigner, I am somehow mistaken (my Chinese is near fluency). I've had many many Chinese outright call me a liar to my face. Or in the case of a heated situation, simply say "Oh, you must have been mistaken. They and or you simply didn't understand what you were talking about.' Its funny people will say that after we've had a very deep conversation purely in Chinese or English. So what they mean is 'everything you said I understood perfectly, except for the parts I deem unworthy of understanding.. because of your poor understanding of Chinese.' Which is BS. I had this happen just last night. The lady I was talking to said everyone understood our entire 2 hour conversation in Chinese, except for the parts that I 'wasn't' lied to about- because of course my Chinese is sub-par. 

I haven't had any problems yet (well, I guess some might consider 6000/month a type of problem), so I've just been happy-go-lucky WASP guy.

I found that if you practice playing with swords or nunchucks, or training with some martial art (and they have seen you), then tend to be far less antagonistic

But Crimochina, saying that something is more common than oxygen in China is not exactly a superlative. More common than CO, CO2 and haze might be a more apt phrase to use.

Neither one works you'd be better off neutral. Thick face and angry would cause for hard times later. Gentle kind hearted they might think you are too giving and will work you to death!

How about ... just saying "no" with a smiley face?? And keep saying 'no' with that smile on your face...

And, then, sometime later, say 'yes' to something you're willing to do, with the same smile. It will be confusing at first, but eventually they'll get the hint that you'll do exactly as you choose!

I usually go with schizophrenic. If I am pissed off or don't want to do something, I damn well show it. If I am not too fussed or happy to go along with it, all smiles.