I've been to Idol 3 times, and fairycat has pretty much covered it.
I don't think they "over-ticket" Idol as much as they do other shows, but I'd still get there at least 2 hours before the arrival time on your ticket to secure a decent spot. If nothing else, the closer you are to the front of the line on the street, the better chance you'll have of having a seat on a bench in the "holding pen" inside the studio lot. (And just so you know: People will have been in line from early in the morning, and frequently they are holding places for several others to join them. Don't be surprised to find that the line in front of you doubles or even triples in size as it gets closer to show time.)
Definitely pay attention to the dress code. You don't necessarily have to have "club wear" but they are pretty serious about No Whites or Logo Shirts. I've seen more than one person have to drape a scarf around their neck or turn their shirt inside out at the last minute to gain entrance. And ditch the sunglasses, too.
They were better at offering restroom facilities when I went earlier this season -- (previously we were restricted to using a couple port-a-potties in the parking lot) -- but you still might not want to drink tons of fluids beforehand.
Once inside the actual studio, the audience handlers will direct you where to sit or stand. They might even split up your group for various reasons. If you have a problem with something, try to be as calm and polite to them as you can. They might seem rather abrubt and harried, but don't take it personally.
Best advice is just to be patient and not stress. You might occasionally feel like you're "cattle", but as long as you can take that in stride, you should have a great time.
