Hey auditioners--
Everyone else has already given you some good advice, but here's my experience:
I did two road auditions in Florida maybe five years ago, passed the tests, seemed to have great interviews, and got the postcards telling me I wasn't in the pool, then after repeating in NYC a few years later, I got the "yes" card and went on to do very well on the show, so if this is really your thing, don't ever give up.

Keeping in mind that all of these experiences are at least a few years old:
The line: At both my road shows, I was there a few minutes before the announced time in the morning, and there were already dozens, if not hundreds, of people already waiting. I don't believe they'd turn you away for being at the end of a long line, but why risk it? I always approached the whole adventure like a second job, prepared meticulously and was always early.

The questions on the test: There were 30 questions, with four possible choices each, just like on the show. I'm an excellent player with a strong memory and a broad range of knowledge, and I did find most of the questions pretty straightforward. However, there were a few where I had to guess between two possibilities, a few where the answer choices seemed ambiguous, and one or two that I knew I just didn't know. (Keep in mind I have a lot of experience with quiz shows, so this is probably not typical. DO NOT assume the test is easy. Most of the people who took it with me said it was hard, and consistently only about 10 percent who take it pass it. Prepare as well as you possibly can.)
Most of the pop culture and current events questions seemed to have been taken from events in the previous year or two. However, I did get some questions about TV shows from a generation or two ago: I think one was about "Full House," of all things.
I have no idea what a passing score is, but on every test I took I never thought I'd missed more than five, if that many. The questions do represent a broad variety of things, including literature, history, science, Nobel Prize and other award winners, geography, pop culture, TV, the usual things you'd expect. I do retain the impression that the tests featured far more U.S. history than world history. I recall being asked about the Civil Rights movement, the previous year's Nobel Peace Prize winner, foreign currency, national parks, chemical elements, Halle Berry and Bob Dole, if that gives you any idea. I personally have never found questions on the tests that I thought were as tricky as the actual show questions above, say, the $100K level.
DO NOT take the test for granted, even if you are very good already. I know I'm good, and I never took anything for granted. Probably the best thing you can do to prepare is go through your World Almanac. And catch up on your diet of TV news and pop culture, including slogans and commercials past and present, which I mention because they do seem to ask about that quite a bit, on the show and in the tests I took over the years. They also seemed to ask quite a few U.S. government things, as in so-and-so is a senator from which state, what is such-and-such specific constitutional amendment about, etc. This is your World Almanac stuff...
I have no idea where they get these questions, or if they're from previous shows, although some people have told me they remember some from the show.
I feel a bit silly bringing this up, but do be very careful to bubble in your answer sheet completely and darkly with the pencil, just like in school, and double check to make sure you've answered the right number question on the answer sheet. You'll have an answer sheet that's separate from your question sheet, and sometimes the layout doesn't correspond between the two in the way you'd intuitively expect. If you have extra time, go back over the questions to make sure your answers are correct. There are no extra points for finishing quickly. I used every bit of extra time to go back over things, and on one occasion I think an answer I changed at the last minute might have been the difference between passing and failing.
The interview: They will probably give you the chance to print out the contestant questionnaire and bring it with you. DO IT! Bringing it there already filled out gives you the chance to think about your responses and organize your thoughts. If you do nothing else I say, think about your personal anecdotes ahead of time and have them on the tip of your tongue. Brainstorm all the unusual/interesting things about you, things that would engage an audience and that you can talk about engagingly, write them all down on scratch paper, then use a different one to fill in each of the questions.
Give very detailed answers on that questionnaire. One producer said she really liked my detailed answers.
I do think that it's more your attitude when you interact with the producer, than the substance of the anecdotes. Few of us have shot tigers in India and skinned them with our bare teeth, or lived in a penguin colony in Antarctica. I certainly thought my anecdotes were zzzzz-inducing.

Just pick the ones that the "average" audience member might like to hear about, the ones that are the most "different," and practice talking about them with as much sparkle as you can. Practice on your friends and family and get them to critique you. Honestly.

And ask your loved ones to give you anecdotes about yourself--you'll be surprised (I hope pleasantly) at what you haven't thought of. Think of cute little one-liners that sum up each anecdote. Comedians will tell you it's all in the delivery.
Write legibly and form your sentences clearly and completely. Let your writing show personality and charm in the way you tell the stories, yet still be concise and don't run on unnecessarily--maybe a paragraph/four or five sentences per question. Be like a politician: If they ask you a question that's less than ideal for you, answer it, but use the answer as a place to jump into something else more interesting. If you're an academic, don't write or speak like one. In your written responses and in the interview, be concise and engaging, and don't use "big words" unless it's for humorous effect. Know when to stop. Knowing that you'll have maybe one minute for the interview, take your two or three best stories and refine them into a concise "elevator pitch" for the oral interview.
On the road, realize you may be interviewed in close quarters. In one of mine, I had to stand in the doorway of a cramped little room in a car dealership, with the producer inches away from me, while people squeezed past us as they walked down a narrow hall. And it was noisy. Don't let anything unexpected throw you, and don't look confused or nervous.
The producer will probably spend no more than one or two minutes with you. The minute you walk toward them, you should be smiling (genuinely, not maniacally).

Your body language should reflect relaxation, friendliness, confidence--if you don't feel like that, practice with friends ahead of time how you're going to do that. Have friends do a one-minute interview with you so you learn to think on your feet. I did this for a friend who was auditioning and she said it helped her a lot.
Because the producer has so little time with you, he or she will probably pick only one or two or possibly three things from your questionnaire to ask about. Be direct and answer the questions asked, but try to segue whatever you're asked into a discussion of the anecdotes you feel are your best. And make sure to be very excited and enthusiastic and tell them how much you enjoy the show. Have a genuine give-and-take conversation with the producer--they really are people--so they see you can interact well with Meredith and the studio audience. Laugh at yourself when it feels appropriate, but don't put yourself down. As in any job interview, show them what you can do for them: Draw an audience that will take you to its heart.
The X factor: Having taken and passed the test and not been picked about six times total (on the road and in NYC) before finally getting to yes, I tried to analyze how that could be, so maybe this will help you, but remember all of this is my own experience and opinion, not inside info: Every time I took the test and didn't get picked, I had the same MO. I was completely myself, which means I didn't show nervousness, and was, I think, outgoing, poised and polite. I always thought the interviews went very well and that the producer liked me a lot--but I didn't get picked. At last I thought: Maybe I'm a little TOO poised and polite.... The time I finally got picked, I did one thing differently: I showed a LOT more excitement and desire. In fact, when they called my number as one of the "passers," I jumped up, clapped my hands and screamed with delight, provoking the producer to say, "I dare anyone to top that."

(This likely works better for girls than guys, but there's got to be a male variant on this that should work...) Then in the one-on-one interview, I showed a lot more enthusiasm and told the producer how much I loved the show. She seemed to really like that, and said something like, "Oh, it's so great that you love it so much..." And quite honestly, that was the only difference in my "yes" audition and my "no" auditions. I dressed the same way and behaved about the same, and had pretty similar anecdotes. The only thing I did differently was make a conscious decision to be a lot more excited and exuberant. Be careful, though: I think they can detect phoniness a mile away, so be yourself, just be sure it's your most positive, enthusiastic, exuberant self possible. That's exactly the way I played it, and I continue to think that's what did it.
Wishing all our auditioners LOTS of broken legs--
Lyn