These are some of the rules I've put together over the many years of heading out to ALA conventions and the weekend geek cons. MegaCon '14 is this weekend...
First rule: make sure you got the tickets to go.
Purchasing advance tickets is sometimes cheaper, and helps you start planning ahead
Second rule: MAKE SURE YOU HAVE THE TICKETS ON YOU WHEN YOU GO.
Last thing you need is to fly out to San Diego for the ALA Midwinter and find out your hotel pass is back on your dresser.
Third rule: If you're planning on staying at a hotel for a weekend, pack enough clothes to last you a week.
Expect the unexpected, such as running out of underwear. Happened to me one convention trip and spent a morning looking for a men's wear shop in downtown Chicago on a Sunday (almost nothing is open in the downtown area on a Sunday).
Forth rule: It wouldn't hurt to get a map of the place you're going to. Especially to figure out where parking is, where food is, where bathroom is.
Fifth rule: Don't exactly overpack (except for the UNDERWEAR). If you're traveling, travel as light as possible.
Do plan on where you'll stash your dirty clothes on the trip back, as well as souvenirs.
Sixth rule: Toothbrush. Bring a toothbrush.
And toothpaste. Remember what I said about how hard it is finding a shop when you get to where you're going...
Seventh rule: If you're just going for a day, backpack for carrying stuff will do.
Toothbrush is optional.
Extra underwear isn't.
Eighth rule: if it's a comic-con or geek festival of some kind, there's nothing wrong with cosplay.
Costuming up is fun, and helps you mingle with the crowd a little bit. Just remember: there will be kids, and very angry parents, and very violent security guards. Dress conservatively. If it's a professional convention, dressing up as a Jedi or Queen Elsa isn't going to go over too well... so REALLY dress conservatively for that.
Ninth rule: Take enough money with you to pay with cash all the toys and trinkets and autographed memorabilia you're bound to collect.
Tenth rule: Rest often.
There's enough walking and running and standing around at every kind of con I've been to. Professional, geeky, personal, public, private... EVERY convention is at a big hall, with distant parking lots and hotels, and waiting for shuttle buses, and waiting for food, and waiting for events, etc. Think of attending a convention akin to attending an endurance race, at least a 5K mini-thon or something. And comic book conventions are packed, shoulder-to-shoulder sometimes. I came back from one convention with a successful weight loss of 9 lbs., I think it was the ALA Chicago one back in 2000. I only lost 4 lbs at the last MegaCon though...
I promise I will survive MegaCon and bring back pictures.