Whether you're going to a comics, toy, anime, book, or Harry Potter convention, the presentation is generally the same: dealers room, panel areas, food court, and possibly an Artist Alley, among other things. Oh, and loads of people who are all into a lot of the same things you are, since you've all opted to go out of your way to attend this nerdy event.
Whether it's your first con or your 20th, there are a few ground rules I think you should keep in mind to ensure that you and the people around you have a great experience.
Decide on your personal schedule before showing up. Often, unless your convention is a small event with just one main room for presentations, you'll likely have to pick and choose which panels, guest signings, performances, and photo ops you really don't want to miss. Make a schedule for yourself based around your key interests and give yourself plenty of wiggle room to get from place to place. If you're attending a rather big convention, chances are you'll want to be at least 20 minutes early for popular panels to ensure you'll get inside. Huge events like San Diego Comic Con are, of course, their own ball game.
Lines suck, but they don't have to be a nightmare for everyone involved. If you're early and assured you'll get a seat, an autograph, a photo, or whatever else, then just chill with the folks around you. Schedule your day so you won't have to rush.
Rehearse what you want to say to guests you'll meet. It happens to everyone. Ya make the line to meet the artist of your favorite comic, an actor in one of your favorite flicks or TV shows, the writer of one of the best books you've read in a while, whoever it is, and when you finally make it to the front, you barely stutter out a "Buh...I LOVE YOUR WORK!" and if you're lucky, stop there, else ramble on incoherently. And I'm sure it's flattering for the guest, but often times when you aren't in the presence of awesome people whose work you love, you've got plenty of great things to say about them.
Try and consider these things ahead of time if you know you're going to make a particular guest's event, maybe even write it down so you can have it at the front of your mind when you're getting to the front of that line, and deliver it like a champ. Tell them what your feelings really are, and let them know exactly what it is you love about their work and how it moves you. They'll love and appreciate you for coming out no matter what, though, so this is just some food for thought.
If you haven't got anything nice to say... Just don't say it! Unless you're witnessing harassment of any kind, there is no reason to be negative or discouraging at a nerd event. Someone's cosplay stinks? Just don't talk about it. Someone's chatting with you about a book you both like, but has a different opinion that you think is WRONG about the events of chapter 10? Let it slide unless you can have a friendly debate. Steer the conversation somewhere else, and bonus points if you can focus instead on something you both like in the same way, or the one bit of the person's costume that's perfectly executed, and roll with it.
Now that we mention harassment...
Seriously, just don't. Don't be that person who harasses a cosplayer or shuts down a fan because they don't know as much trivia about your nerd-dom as you do. Don't touch people inappropriately or hug without permission (no matter what they're wearing), don't be a jerk to guests, and especially don't berate con staff, who tend to be volunteers.
Additionally, if you see bad behavior from another con-goer, you should intervene. If you don't want to get directly involved, flag down a staff member and tell them what's up. Conventions should be a positive and safe experience for all attendees, and your active involvement in even "small" matters can go a long way.
You're not the only person at the guest panel. When there's a guest panel, there tends to be an open Q&A in which attendees can ask their own questions. Some attendees use that opportunity to talk about how much they love the guest and thank them for whatever-- look, we know.
Everyone's there because they think the guest is awesome, too! Don't waste precious time complimenting the guests during the panel when the point is to ask fun and/or interesting questions. Engage the guest(s) in a productive way. Bonus points if you research a little bit and come up with a question that hasn't been answered previously in some other interview or event.
The time for complimenting the guest is when you go to their signing, catch them after the panel or at their table, see them walking around, or attend their photo-op, not when you've got the mic while 50 other people have questions they want to ask, too.
If there are more freebies to be had than you can carry... don't try. I'm going to reiterate my advice from Free Comic Book Day and say that you really shouldn't just grab every free thing in sight. Pick out the things you know you'll use/read/play/display/etc. and spare yourself the clutter and added pack-on weight.
This goes double for book con attendees who are seeing the publisher booths stacked high with samples and calculating how many they'll be able to carry back to the hotel. It isn't a contest - make it meaningful and leave the stuff you aren't particularly interested in for other attendees to grab.
Carry cash in small denominations. The easiest way to buy all the awesome merch you want at your event is to use cash, though many exhibitors will take credit these days. There are two problems with credit, though. First, it's slower, despite typically being the quicker option at a department store. Second, the signal probably stinks at the convention center, meaning it might take the merchant or attendant several tries to get the transaction through, which ties in to my first point about credit being slower. Carrying around a wad of small cash is just way faster and easier.
With these con-going basics in mind, you should be well on your way to a happy weekend at an awesome nerd event. As usual, the key thing is to be considerate of others, but at the same time, you should focus on maximizing your enjoyment by having a game plan. And don't forget to take tons of pictures!