It's a big show, so go for as many days as you can. I did some back-of-an-envelope maths once and based on there being approximately 400 cars and bikes, and you went for 8 hours a day for 3 days, you could spend no more than 3 minutes 36 seconds at each vehicle if you want to see them all - and that doesn't include the manufacturer stands, moving around, eating, getting autographs or anything else. I tend to go for all 4 days, getting there as early as possible and leaving as late as possible, mostly because the best photos are to be had when no-one's there to get in the way. Then again, I suffer from a horrible affliction of wanting to see absolutely everything, so if you're more relaxed than I am about missing things then you don't have to be there for so long.
Wear comfy shoes. The hillclimb is over a mile long uphill (obviously), so if you want to see the rally stage at the top of the hill (and you should) then you'll do plenty of walking. The rally stage itself is a chalk surface, and in the dry gets very dusty.
Everything is expensive, so I take my own food and drink and not buy much. There's a good range of stalls to wander round in the marketplace and if you buy something large and/or heavy I've never come across one that won't hold on to an item you've bought so you can pick it up at the end of the day. This might sound obvious but it always amazes me how many people don't think to ask and end up struggling to carry their purchases with them.
Take plenty of batteries for your camera, and a few spare SD cards. If you're anything like me then you'll be taking lots of pictures.
Unless you stay in a hotel on Goodwood Estate somewhere you'll be battling the rush hour traffic on Friday morning and evening, so be aware.
The Moving Motorshow on the Thursday is like a Festival lite; the paddocks with the race cars are open, although only the road based cars run. It's usually not as busy so good for getting your bearings at your first FoS.
The best places to watch the hillclimb itself are opposite Goodwood House for donuts and burnouts, Molecomb (an off-camber left hand corner halfway up the hill - there's usually at least 2 cars in the bales there by the end of the weekend) and the top paddock (more donuts). As all that seperates you from cars doing 140mph+ is some straw bales and a length of rope, you'll get a good view of the cars from any of the spectator areas. I've never bought a grandstand ticket so I can't comment on the views from them.
Take ear plugs/defenders if loud noises bother you. When you've stood 3 feet from the exhaust of the Pagani Zonda R blipping it's throttle in a marquee you know about it for some time afterwards.
When walking through the paddocks, watch out for cars coming in and out. It might be exiting seeing Senna's McLaren or a multi-million pound Alfa Romeo driving past you but you won't want to be the one who breaks it by falling on it, not to mention the damage you'll do to yourself.
I know there's a lot of text there but it's all things I've found from years of experience at the event. It really is mind blowing just what turns up every year and more to the point, just how close you can get to the cars and drivers. I've managed to find the likes of Jenson Button, Colin McRae and Richard Petty just wandering through the event and most of the drivers are happy to sign and autograph and have a quick chat.