As someone who has been to the Bangkok Motorshow 3 times in a row here's some advise:
1 - Use an external flash to give you more reach. Almost every carmaker will have a performance where there will be dancers and performers doing a routine. You will be standing at least 10 metres from them so the external flash will be an advantage if you wish to take pictures. After the performance you can go closer.
2 - The car show is HUGE so come early like 1 pm so you can take your time to explore the exhibition halls There will be halls for popular cars like Toyota, Nissan etc, hall for trucks and good vehicles, hall for motorcycles and another hall dedicated for car accessories like upholstery, solar film, car audio etc.
3. If you like crowds go on a weekend. If not, go on a weekday to take pictures with few obstructions. However even on a weekday it will start to get crowded around 4pm onwards.
4. Every car maker will have their stable of girls, around 5 to 10 girls. The luxury marques like Mercedes and BMW will outfit their girls will elegant dresses. The japanese makers will have their girls in typical japanese outfits like what we've seen in Singapore motor show. The motorcycle girls wear casual fun clothes. All the girls will take turns to speak (in Thai of course) to promote the car models. Do not ignore the accessories hall as they too have pretty promoters.
5. Around 4pm the car makers roll out their performances where you will see dancers or acrobats. Since there so many exhibitors there the challenge is to cover as many as you can. Usually after a show end another one starts nearby by another car maker.
6. Stay until after sunset and check out the car audio exhibition outside BITEC. There you will find vehicles souped up with multiple speakers and subwoofers. Also you can see some coyote dancers performing on stage while the cars and vans pump up the Thai disco rhythm. Needless to say flash is required!
Sorry for the long post.