1

Two choices:
1) Leave China by air, land, or sea at any border, get stamped into another country, turn around and come back to China to get stamped in and start the clock on your second 90 days.

2) Go to PSB and see if they will give you an extension. You will need your zanzhuzheng (Certificate of Temporary Residence) slip of paper from the Police. If in a hotel/hostel, they can give you this as they already registered you. If you are in private lodging, you should have gotten this yourself.

Risk of Option 2 is that they could refuse the extension altogether (on a 90 day duration, possible) OR, give you a 30 day extension and also cancel the second entry altogether (possible and in some locations, probable). Note that extentions are not free--it will usually cost you the same as getting a new visa.

Strategy to pick depends on your nationality (determines extension cost), how close you are to a border and travel costs to get there, and what your future plans are...will you need the second entry or not? If you definitely need the second entry, then I usually recommend Option 1. In which case, it would be helpful to know your passport country and current location, as some neighboring countries might have no visa requirement for you, eliminating one level of cost/hassle.

2

Note that you only need to leave and re-enter Mainland China if you choose option 1, so you can exit to Macau and Hong Kong and then return.

3

Or exit to N. Korea and see the missle launch. Two birds with one ....

4

As of last august, they (PSB) weren't extending multiple entry visas. Or rather they were, but it required invalidating the original, i.e. cancelling any remaining entries and issuing a new one, with a new fee.

5

My experience has been the same as foolishmortal's- they won't technically extend the visa, but cancel it and give a new visa for thirty days without multiple entries. Last year in Shanghai you could do it twice before having to leave and get a new L visa (or other kind of visa).