While reading "Using on camera flash in banquet hall with high ceilings", I realized that I have a similar problem at hand.
I regularly (at least twice a year) shoot conferences for a large company.
These are held in a wide but narrow room (I would guess
15 * 45 * 3m (L*W*H) with one wooden wall (think of walnut), and three yellow-colored leathery walls, a dark floor (think of dark oak) and a black, moderately reflective ceiling. One side features 1m high, shaded windows, the ceiling's lights are small HQI-lamps, while the speaker's desks are lit by tungsten PAR64 (50% output at best).
Photo of the location (from front to back). 24mm, 1/125s | f/5.6 | ISO 6400, E-TTL flash fired (angled via ceiling). I am standing next to the wooden wall and 5m in front of the stage. Notice the dark-semi-reflective ceiling and the yellow walls.
Without flash, ISO 6400 @ 1/100s and f/5.6 is underexposed by around 1-2 EV (depends on the exact location). 1/80 or faster are needed, as I need to stop peoples' motions, and f/4 or narrower is needed (often), as most pictures feature groups of people and/or a background that should be discernible.
I know that this is a very specific description of the situation - I thought that it would be better to offer specific conditions and get general answers than having it vice versa.
My solution is to angle the hotshoe-flash (GN 52) slightly towards the subject (so not straight upwards) and using a large bounce card (out of paper for now, but I plan to design one myself with oblique white arylic glass). For group pictures, I use multiple flashes off-camera with a similar setup. For pictures of the PAR64-lit area, I use a CTO gel.
Generally, I use manual mode, but leave the flash at E-TTL and only play around with the flash exposure compensation.
The flash offers fairly little compensation - it needs very high power levels to offer any support and because of the reflectiveness of the ceiling, it does not offer a soft light. This means that while the flash offers a bit more light, it does little for eye-shadows.
My solution works - my employer is happy with the photos (at least that is what they say), but to me, the pictures are too noisy and too flat.
Aside from asking the room's technician to give me as much ambient light as possible (and perhaps using CTB-gels for the PAR64s), are there other improvements to my setup or other solutions for this kind of situation that I have missed so far?