Rob (Mr Fish), if your using a long shutter speed to create light trails it will make it far easier to use first instead of second curtain flash, exposure and image will be the same but you'll be able to see the crop/where people will appear in the end photo instead of having to estimate what speed you need to move your camera at.
Also generally speaking using manual flash instead of ETTL is far more accurate and saves you messing around with FEC depending on what people are wearing. That said the 7D that I just got is far better at ETTL than my 40D was so it might depend on how new your camera is (manual is still better though). It also isn't hard to do because in a nightclub you dont vary the distance you are from people very much. Probably about 95% of my shots are done with a 1 stop change in flash exposure or less.
tomcraven-As has been said before that's not moisture in your photos, its underexposure. Add +1 FEC to your shots and I reckon it should be pretty close to spot on. I also don't know what height you are but its better to get down to the same height as the people you are photographing (in your 3rd shot your looking down on the girls)
I appreciate everyone has their own style and technique but I disagree strongly with most of your post. Light trails I can understand, they add a funky effect although it's not my sort of thing, and our customer base also doesn't call for it.
As for using manual flash...don't even get me in to that. Me and my boss were having a chat about this a few nights back, and we were talking about how this company would most likely not exist if it were not for things such as digital, auto focus, ETTL etc. I joked about going for a whole night without using AF and we both decided it simply would not work. Although I do feel in a way that all this auto stuff is taking the job away from the camera man, when you're taking 1500+ shots in a nights work, you rely on things like ETTL and AF because you simply do not have time to do it yourself.
FYI we use M mode at wide open aperture (2.8 or 3.5 depending on which lens), shutter speed normally ALWAYS at 1/20 although you can go up or down a couple of stops if you have a venue with significantly better/worse lighting. I try to avoid going slower than 1/10 because you start getting signs of slow sync in the background and sometimes around your subjects. ISO stays fixed at 800 but we NEVER use manual focus or manual flash. It's ok if your subjects are always at the same distance for you, but there's just too much volatility in the scene in my type of work for me to mess about with flash settings. Plus I don't always use direct flash, I might bounce off a wall, use direct with a stofen, indirect with a stofen, bounce with a stofen etc.
We don't even shoot in RAW because it takes too much time to process afterwards. Everything is done in JPEG small (or medium on our club camera since they have less megapixels).
WAY too tight for nightclub shooting - you'd never get the clear space and large groups are out of the question.
Buy a Sigma 10-20mm - it's perfect for this work.
I absolutely agree that 50mm is ridiculous for nightclub work. I rarely find myself moving away from the wide end of a 16-35 or 18-55, I don't know how the hell you could shoot at 50mm!
However I wouldn't use a Sigma 10-20mm either, I think they are f/4 at the wide end and go to f/5.6 at the tight end? Sometimes I find f/3.5 a pain in the arse, especially when you're in a place with dark walls as the flash has to work harder. f/2.8 is ideal especially on a lens with constant max aperture like the 16-35.