about multiple entry visa

Hi,
I am from the USA and have a multiple visa entry 60 days. I now plan on looking to overstay my 60 days as it is almost up. My question is from Shanghai is flying to Hong Kong my best bet in terms of savings $ and time?

Also, if i fly to Hong Kong, exit customs, then turn right back around and go through customs to take the next available flight back to Shanghai? Is this possible or am i required to say abroad for a certain amount of days or hours in order to re-up my 60 days?