Do lots of research. You really do need to know a lot about DC - it is like nothing else
Be prepared for the bills - our trip ended up costing $8000 for the two of us (staying at the Marriott) ... and we didn't even spend that much in the dealer halls!
The con is held between 4 hotels - if you can, stay at one of them. They are a little more expensive, but it's money well spent to be right there. The Marriott is the one nearly everyone tries to stay at - but it sold out in 14 minutes
this year, but I hear one or two of the others might
still have rooms. There are lots of other hotels close by, but you will have to walk (the main hotels are joined by skywalks, so you never have to go outside in costume). The host hotels also have 'DC TV' - live and replays from the panels you might have missed - including the costume contests :)
It goes for 24 hours. The dealer halls close in the evening, but most head to the Marriott lobby to socialise. We went to bed at 6am, and the party was still going.
It technically starts on Friday, but badge pick up is on Thursday - so once everyone arrives to get their badges, the costumes come out :) Monday is a lot quieter, as a lot of Americans have to be back at work on the Tuesday (Monday is a public holiday, so most shops will be closed). If you can, fly a day or two earlier - catch up on sleep/adjust to the new time zone, it can really ruin the weekend if you're tired (it's hard knowing every time you sleep, you're missing it!)
The usual things people suggest at big cons - vitamins (con crud is a factor with that many people), shower at least once a day (you're basically living in costume for 4 days), eat at least 3 times a day, keep hydrated (the Marriott has a 24 hour food store) - the drinking age is 21 in Atlanta, and alcohol is not sold on Sundays (hence the popularity of room parties)... plasters, pain killers, costume repair kits, inner soles for long days in costume shoes... and other US generic things - like tax, tips etc.
The weather is very warm in Atlanta at that time of the year - if you're taking big costumes, be prepared to sweat. A lot. A little better inside, but air conditioning doesn't do much when there's that many other people all hot in costumes too.. hence the hydration/showering tip.
Don't waste money on taxis - the hotels are a quick train trip from the Atlanta airport. The MARTA train system only costs a couple of dollars, and takes you right to the Peachtree centre, connected to the hotels - very easy to use.
The elevators will always be a problem with that many people, just be patient. Some people request a low floor for their room, but I don't think it makes much difference (we stayed on the 35th).
Lines. Always a big problem at such large conventions. If there is a major guest you want to see, be prepared to camp out for at least 2 hours (eg. William Shatner/Leonard Nimmoy - 2 to 4 hours in line to make sure you got in the room). Some panels you can just walk in, but for the film guests, be prepared to wait. The costs for photos/autographs can vary a lot too - starting at US$30 for some, all the way up to US$200 (Patrick Stewart).
Be prepared to miss things you want to see. The final schedule is posted a few days prior, and you'll find lots of things you want to see will be at the same time (or clash with fan events). If you relax and just plan as you go, you won't spend the weekend venting about missing that one panel... There is just a huge amount going on at any one time, there is always something to enjoy :) The major panels/contests are filmed and released on dvd after the event, so you can always view them later.
Have your badge with you at all times - secure it on a good lanyard. Even in costume - wear it underneath, or in a secure pocket. If you lose it, you will have to buy another one (about US$100 to buy at the con). Remember to take photo id with you on badge pick up day, or you'll spend 4 hours in line to be told to come back with id...
The parade - the must do (unless you partied a little too much the night before) event for DC. If you want to march, you must sign up with a registered group (or register your own group), and collect a participant ribbon on that morning to stick on your badge. Half of Atlanta turns out to watch, it is a main feature of the weekend.
Anime/manga isn't big at DragonCon - you will see a few, but with so many solely anime US conventions, DC is mostly movie/fantasy/comic book type stuff (Star Wars always has a big presence). There is also a big goth/punk scene. Then there are a fair amount who attend for the science/literary panels.
There is a 'DragonCon After Dark' atmosphere late at night - definitely not appropriate for children. Stickers on nipples and duct tape over privates is not uncommon. It's not 'out in the open', but just be prepared that it is there. If you've ever searched for photos online, you'll know what I mean. It isn't a 'family event' in the sense that Armageddon is - you see a few infants, but very few children. A number of reasons, but it is more of a 'grown up' party :)
The DragonCon LiveJournal community
is great for anyone planning to go - heaps of helpful posts :) They also keep an updated list of all the different group photoshoots - which is great if you want to join in or just take photos :)
The memberships (tickets) are available to purchase from the DC website - they increase in price by $10 every couple of months (they start at US$60, they've already gone up to $70
Phew! That ended up being longer than I intended. Let me know if there's anything else I haven't covered