Having been to the Geneva Motor Show for a few years running, and having also attended other similar events, I thought I would compile a collection of thoughts on what to think about when you want to take pictures at such events.
I try to avoid weekends, as they are a lot more crowded than weekdays. Sure, it means taking a day off work if you're not a professional, but weekends (and rainy Sundays in particular) are a no-no.
Even better, if you have some connections and get yourself into a press day, you'll have a lot more space, a lot more access to cars that are not available to the general public, and more models "draped" over the cars for that additional glamour touch.
If that's not an option, making sure you pass through the gates as soon as the show opens generally means you have at least one hour before it gets crowded. The other option is visiting in the last couple of hours before closing time, when the crowds start to leave. The disadvantages in this case is that the people on the stands will look tired, and the cars may be quite dirty with fingerprints.
Make a plan of what you want to shoot before you go. Know where the manufacturers you're most interested in are, know what will be on display (especially if you're interested in premieres and concept cars, as I am).
Some stands will get very crowded very quickly. Go there first.
Light at motor shows is a mixed bag. There's not a lot, and most of it comes from spotlights that will lead to a million reflections on the cars. In any case, a camera with good low-light capabilities is a must. 800 ISO is the norm, so if you can't shoot without (too much) noise at that setting, your pictures will not be good.
You will need a flash, but since most of the time you'll be very close to the vehicle, it will be mostly to fill in shadows. Get yourself one of those white diffusers that fit on the flash. In my opinion, there's no need to use one of those "Whale Tail" ones, as they are only useful when there are walls to bounce the light against, and in a motor show most of the time there won't be walls available.
Having a wide angle objective is a must for whole-car shots. I use a 16-35 on an APS-C camera. A 24-70 on a full-frame is about ideal. If you're more interested in the hostesses than in the cars, then a 70-200 zoom (2.8 better than 4.0) should work well.
A macro for car details is always something nice to have around.
If it's the cars you're after, a circular polarizer works wonders in removing reflections on windshields (but takes away a couple of stops of light, and adds glass on top of your lens, so it's a tradeoff between practicality and quality, as usual).
Using a tripod is up to personal preference. On a public day I would not do it, you'll piss people off and there's a good chance your camera will get knocked over by someone not paying attention. Tripods are very useful to take multiple shots of the same car to combine them and remove people, but again that's a lot of work, and if the light setup changes over time the result may not be up to your expectations.
The more professional your equipment looks (and that's becoming harder and harder in these days when everyone has a camera at this type of event), and the more professional your attitude is, the more likely you are to get access stands where the general public is not allowed. Be polite and patient, and you'd be amazed by how many big name brands will give you access for a few minutes. Again, this is easier if there are not a gazillion people around, as they would all be screaming "me too" when they see you walk in.
You can't do 5 hours straight without sitting down. Take a short break every 90 minutes or so. Drink. Wear light clothes. If you're so inclined, you can take your laptop to work on some shots during a break, there are in general enough areas where the public can sit down and relax for a while.
Be patient! There will be times when you will have to wait quite a long time to be able to shoot a car without people in the way, or with the right background (especially true for cars that are in front of a screen). Also, when there are a lot of people, while you're waiting for your turn think about what type of shot you will be able to take, notice where the light sources are, etc. Chat with your neighbor while waiting, who knows, it may open doors in another area of the show!
A final note on shooting a car rotating on a plinth: don't forget that it's moving! Anything less than 1/200th of a second will be blurred, especially if you're shooting from very close with a wide-angle. It's an easy enough mistake to make (been there, done that!)