According to Chinese mentality, it is indeed considered impolite to refuse a cigarette.

Some will say, take it and give it to somebody else later...

As for me, I always say that my doctor is tough and forbade me to smoke. Then the guy will feel that he is not losing face and everything is fine.

It's like drinking... if you say you can't then they will let it go. But if you make the mistake and drink the first cup, then you're stuck taking each one after that.

I don't think it's impolite. Well, maybe it's different if you're a female, I don't know. Whenever I get offered a cigarette I politely tell them that I don't smoke. Everyone's always been good about it so far. I've even seen Chinese people refusing cigrattes. I think in this day and age, everyone knows that smoking causes cancer and so, everyone respects non-smokers.

I dont smoke either, so i just smile and waive the offer off. Never been a problem.

I never refuse, I think this is very kind of them to offer it

it is not rude. you just say you cannot smoke. that is all right.

Indeed it is not rude.  Things are changing in China -- I have known a good number of guys that have given up smoking.  In my place of employment, the management puts Chinese-style pressure on the male employees to quit smoking.  Many have, several not.

It is not impolite to refuse a cigarette in China nor is it impolite to refuse a drink.  No, thank you, I don't smoke and no, thank you, I don't drink, that suffices.  If it's drinking, then just turn your glass upside down or fill it with tea or water.

That's called peer pressure. If someone tells you to drink alcohol do you do it? If someone tells you to jump off a cliff do you listen to them? So no it wouldn't be rude to decline a cigarette especially if your not a smoker and they know it. If they know your not a smoker and they offer you one than that is rude.

"Bu hui"... says you don't smoke, and also says it's a social norm that you don't have to accept.

If you are a smoker and refuse, then yes this is rude.
Often I am given smokes by people even if I am already smoking one, and I almost always accept just to be polite and so they won't lose face.
For non smokers (especially men), when they offer a smoke, it is because most men in China smoke and they assume you do too.  If you tell them you don't smoke then no face is lost.  
I even know many Chinese men that don't smoke but will on occasion have 1 or 2 especially when drinking.  Smoking in China is a very social thing much like drinking.

I find that I have to carry two packets; the more expensive Western brand I prefer, and a lighter local one. It's no problem to refuse, even if you do smoke. The social aspect of it is really quite nice, like swapping Pokemon cards.  However, the politics of accepting a light are a little more nuanced...

yeah, it is rude to refuse.
if u dont want the cigarette,just say you cannot smoke.