Developer I have been going near on 10 years to the Salon. Incredible value for money at 16CHF/£12. Best way to do it is in and out in a day. Except for this first time you will need two days
87th Geneva International Motor Show
I would fly out on an early flight, (stay in a airport hotel if your not near) and fly back that evening. Tuesday, Wednesday or Thursday so you don't catch the ski traffic, definitely not Saturday (ski holiday transfer day). Hotels are a fortune and sometimes unobtainable. But the IBIS below is a 15 minute walk if you want a hotel. Usually it's the last reasonably priced one left.
Cheap hotel GENEVA - Ibis budget Geneve Aeroport
(Saying that I'm getting the train home this year - Leave Geneva central just before 10, arrive in London just before 5. This is the most chilled way getting back).
Access wise it's easy, walk out of Geneva airport customs, turn left, head for train station, keep walking. 10 minutes (total) and your at the Palexpo, the first hall you come too is the aftermarket servicing hall 7 (which in itself is interesting) walk through it to the escalators at the back - that will take you to the main 6(!) halls.
http://www.gims.swiss/media/document/1/plan_des_marques_06_02_17.pdf
Decide before your priorities for visiting - you will only scratch the surface. Forget Ferrari, Lamborghini and any stands oppposite them - it's a bun fight! Eating and drinking it traditional Swiss prices, there is (shock horror) a McDonalds and a Starbucks in the Airport forcourt now (turn left not right out of customs). Personally stuff yourself airside in England and on the plane, then just have drinks as you need them. Check your coat in, shirt and trousers are all you need, small wallet, phone. Zip the rest in your coat, even the passports. Don't bother brochure hunting - it doesn't exist these days.
Your day will go quick, very quick. You will enjoy every minute