above and beyond that..
I'll only use direct (face on) outdoors and then only if I'm trying to add a touch of fill flash (-2 stops or less on the flash ) or aiming to get a catch light in the eyes.
other than that I'd bounce it off the ceiling or a reflector or use a defuser for a "bulb" effect.
Bouncing off of a ceiling requires a fairly low ceiling with a light neutral tone (too high and the flash's light is lost, too dark light is lost, not neutral it then gives a colour cast)
I can't think of a single nightclub that I've haunted that would meet even a single one of these criteria (though a black ceiling is technically a "neutral" tone)
As such you'll need to bounce or defuse the light another way..
I have a Lumiquest "big bounce"
http://www.lumiquest.com/lq881.htm which is a reflector/difuser and it's great though a bit cumbersome they have smaller ones (including "pocketable" ones and other company's make similar products.
a Sofen (or similar) diffuser is also good, the effect is different in that it mixes direct light with a small amount of reflected light from 360deg (so long as theirs a surface for it to bounce off)... gives an appearance like the old chemical "flash bulb" ... think early years paparazzi shots.
A third alternative (that also can be used to augment the two other alternatives above (reflector or defuser) is a flash bracket. How this helps with the reflector is that when you tilt the camera in to portrait mode the flash and it's reflector would normally no longer be mounted above the camera but instead to the side, using a bracket with a flip arm allows the flash to be rotated back up in to the horizontal position
With the flash higher (and offset to one side) you do lose a bit of the harshness of direct flash... you will still get hard shadows but red-eye is practically eliminated (again you want to stop down the flash).
don't get a cheap nasty flash bracket (or get one to see if it helps then replace it as soon as you know your going to us it) the "Custom Brackets" and "Really Right Stuff" flash brackets are great but, honestly, total overkill for what you need.
I've got the Stroboframe T-Press bracket and although it makes gripping the camera in portrait mode a bit awkward it's not as bulky or dorky looking as the Custom Bracket or Really Right Stuff setups (and is quite a bit cheaper).
also in a place like a nightclub you'll probably want the camera to pick up more ambient (background) light than the camera's auto exposure program thinks you want - this is so you pick up some of the room so that the subject apears in some sort of context rather than a single person in a dark room. Work in Shutter or Program mode rather than one of the preset modes.
I usually dial flash compensation down 1 or 2 stops and set second curtain sync on the camera. Use a fast lens (2.8 or faster preferably 1.8 or faster) to help compensate for the lower output from the flash and draw in more ambient light. A fast (f2.8 ) lens with IS can be great here if your subjects are static (such as posed portraits) as it allows moving people in the background to blur out (hence the second curtain sync) which can give an interesting, though not always pleasing, effect.
You'll probably want to chimp the preview a bit and ride the exposure (non flash) compensation dial a bit until you learn by heart what exposure compensation you'll need for each specific shot.