I can't speak for China, but I often travel between the two countries that I am a citizen of (Australia and the UK) with no problems.

Basically, at each stage, think about what the passport check is for. So:

• at check-in when leaving the US, you need to show your HKSAR passport (because the airline wants to ensure that you are admissible at the other end of the trip; see also below)
• at security checks in the US, best to show your US passport (see below)
• at the aircraft door, show your HKSAR passport (again, this is the airline checking)
• on arrival in China, show your HKSAR passport (obviously)
• on checking in to return to the USA, show your US passport (same reasons as above...)
• at passport control on leaving China, show your HKSAR passport
• at the aircraft door, show your US passport
• on arrival at the USA, show your US passport

A few extra points:

• US law requires that US citizens must present themselves to US government officials at all times as a US citizen
• therefore, as a US citizen, you must enter the USA on a US passport
• I don't know if the security staff at US airports are US government employees or not, but you might as well show your US passport to them (these are the people, for instance, who conduct the security checks at airports)
• airline check-in staff, however, are not US government employees; however, because they conduct exit immigration control from the USA and because you will not have any evidence in your passport, you might need to explain to them that you are also a US citizen; but I wouldn't volunteer that information until asked, not because you are doing anything wrong, but because some such people get easily confused when something happens that they don't expect
• I don't know what view China takes of people having dual US—HKSAR citizenship; assuming that they are happy with it, then there is no problem
• I have never had anyone question any lack of stamps in my passports, but again I don't know if Chinese immigration people would look for US stamps in your passport —.I suspect not, but if they do and ask you about it, you can always be completely truthful and say that you are also a US citizen (assuming, again, that this is acceptable to China!)