I've been living here almost two years and in terms of feeling safe from being asaulted or robbed, I feel perfectly safe wandering around at 2am by myself in Shanghai. The only violence I've seen has been disputes between drunken businessmen and people fighting with taxi drivers after being over-charged. It really is one of the safest cities to visit at night
Being conned/ripped-off is a competely different story. Be on your guard at all times. Use what you are told by business employees / girls / business owners as information but don't rely on it. Scams are rife here. Never follow anybody or even talk to anybody who approaches you unsolicited on Nanjing Dong Lu and speaks English to you.
Make a night of it. Have a few drinks, wander around for an hour and go past the shop windows. See a girl you're interested in. Maybe keep walking and check out some other shops for something better (you can always go back in an hour's time). Tell them what you want and be clear that you've only got, say Y200 with you (keep Y100 spare in another pocket for a taxi/emergency).
Unlike the scam artists (justifiably) mentioned throughout this forum, the girls know why you're there. You know why you're there. There's no bullshit - except be alert for a second girl to come in uninvited and be very persistent that you do not want to pay double for both (unless you do want that!).
At two different shops I've been specifically negotiated Y50 (about $7) for amassage with happy ending. Both times I've received a half-hearted five minute massage and been told by the girl she is happy to continue but Y50 is just for normal massage and I need to pay another Y50 for the happy ending! I'm sure it's BS and I'm not assertive enough, but for $15 all-up it's no big deal and you can walk out smiling, knowing it's been a safe, scam-free transaction.
One last word. This type of thing is illegal in China, although many shops are in the vicinity of police stations. Police walk past daily and surely know what is going on. It's an open secret, I guess you could say. This said, please look around before going in and don't stroll into one of these places if a police officer is twenty feet away!
Goodluck!