I don't negotiate with drivers on the street. Instead, if I have a long fare (like to the airport, or across the harbor in the middle of the night), then I just call one of those "20%-off gangs" and arrange for pickups.

For visitors who are staying at a hotel and don't speak Cantonese, they can still ask the hotel concierge to do that for them. Much better than negotiating on the street, because those who drive around are not "20%-off gangs", and they need the full fare to make ends meet.

Going to the airport is an exception. Many drivers are willing to give you a discount to take you there, even though they're not part of a "20%-off gang".

BTW, in general, the "20%-off gang" gives you 20% off the meter for medium-haul. 30% for longhaul (HK$150 or more). Full toll, but no luggage fees. No tips necessary either. If one uses a particular driver or "gang" often enough, 40% to/from airport is possible. My cousin is a pilot with CX, and he gets 40% off from his usual "gang" between HKG and Ho Man Tin.

Anyways, my point is that there are two types of taxi operations in Hong Kong. Gang and non-gang. They have different cost, different clientele, different operations. Negotiating on the street is to force a non-gang member takes a pay cut to compete with the gang members. I don't think that's fair.