Regarding visas... If you have an F visa, there are agencies that can extend your visa for 6 mo. for 2500RMB.
However, if you have multiple entry F visa with 90 day duration of stay for each visit, a run to HK is probably your best bet.
If you want to save money, you can go by train straight from BJ. One way train ticket to HK from BJ in hard berth (6 bunks per cabin [3 bunks on each side] and cabins open to the corridor) is 504RMB according to
http://search.huochepiao.com/chaxun/...?txtCheCi=T97B, but I just bought a ticket today at Beijing West station for 454RMB. The train takes about 24 hours, leaving at 1:08pm and arriving at 12:56pm the following day. If you want a more comfortable trip, soft berths are ~800RMB. They have only 2 bunk beds on each side of the cabin, and the cabin is closed to the corridor, i.e. more private.