WWDC is an amazing experience and an incredible opportunity for developers. It's so exciting to be around all of the other great members of the iOS and Mac development community who are just as excited about the state of the platform as you are. With such an exciting event I feel that it's important to plan ahead in order to make the most of the opportunity. Below are some notes on how to be prepared to get the most of the conference.
1) Prepare lab questions along with sample code. If you've got a specific question consider creating a sample project that effectively explains your issue. You want to be able to explain your problem to an Apple Engineer as quickly as possible, so be sure to think about the best way to get your point across.
2) If you're interested in attending the UI Review lab go straight upstairs to the 3rd floor in the morning to sign up for a spot. All of the other labs are first-come-first-serve but the UI and App Store Review labs are scheduled time slots.
3) Plan on arriving to sessions (especially popular ones) at least 10-15 minutes early. This isn't just to get a good seat, but for some sessions it may be just to get in at all. Ushers will close the doors to some sessions when they are completely full, so if you're really interested in attending a specific session plan to arrive early.
4) If a session isn't interesting to you, leave. You're there to get as much out of the conference as possible and if you're not getting anything out of a session you should walk out and head to another one or go to a lab. No one will be offended if you walk out.
5) Talk to other developers. If you are standing next to people you don't recognize then try to introduce yourself. If you see people whose work you admire, tell them. If you see people you recognize from the internet but haven't met, go up and introduce yourself. The Mac/iOS community is so great and everyone there will be very friendly and willing to talk to you if you make the effort.
6) Bring a card with your Twitter name on it. Twitter seems to be the best way for developers to stay in touch since it's something that everyone uses. I still keep up with several developers I met last year via Twitter. Paper cards do seem outdated but they still are the most effective way to pass out contact information at a conference.
7) Choose your seats wisely. Seats near the isle usually have power strips. Try to sit near the front or the center if possible. I usually tried to sit near the isles because I wanted to have an easy exit to get a good seat for the next session.
8) If a session is really interesting or important to you go up and talk to the presenter afterwards. They usually hang out for 10-15 minutes to answer questions, and that can be a great chance to ask a follow-up question if you have one. I got a couple good ARC questions answered after one of the Objective-C sessions last year as well as a CoreData question so I'm really glad I did that.
9) Apple will almost certainly provide beta versions of Mountain Lion, iOS 6, and possibly any other new SDKs the first day of the conference. Make sure you're prepared to receive them. The tables in the cafeteria are all outfitted with ethernet cable drops, which is what you'll need to use to download the files. If you have a MacBook Air, make sure and bring your ethernet adapter. I also recommend setting up your laptop with a separate partition for beta SDKs, especially on the off chance that the next version of Xcode requires Mountain Lion. Last year I set up my iPad and iPhone to sync to my work laptop (not trivial) but hopefully this year that won't be necessary thanks to iCloud. Time will tell.
10) Go to parties (duh)! Half the fun of the conference is going to hang out with the other developers. Go enjoy the atmosphere and meet new friends.
Bonus) The brown liquid substance served between morning sessions in the foyer may be labeled as Coffee and may even closely resemble Coffee, but I assure you that compared to Blue Bottle Coffee it is not Coffee. Blue Bottle is only 2 blocks away, and a great way to step out and get a refreshing beverage (or a great way to start the day out if you spent too much time on number 10).