The only opportunity someone took to try to rip me off was one of the (official) taxi drivers in one late night. When taxis are rare, the drivers seem to very quickly adapt the capitalistic principle of supply and demand to make their money. After some hard negotiations (and the first driver declining my final offer), we agreed on a price of RMB 120 (which would, at daytime, have been half or even less than that). When he finally delivered me at my hotel, he asked for RMB 140. As I knew he was already earning much more than he should on that ride, I nailed him to the RMB 120 we had agreed upon with just a single gesture; obviously I didn't look like I was up to much discussion and he instantly surrendered his request.

At least in these cases it seems that it may be very helpful to instantly clarify that you are not willing to perform any kind of (further) negotiation. If they understand english, just state your offer, say "take it or leave it", hand them the money, and you'll most likely succeed.