Oh my, where to begin. Let's start with before you even get into the convention center!

Point 1: Hygiene and Setting up
One of the big problems in conventions is the smell. It is usually a very good idea to shower and put on deoderant at least every morning before you enter the convention. I don't care how busy or tired you are, it's necessary. In addition, I advise you eat a solid diet. This means you eat things you know are good and don't be too adventurous; you want your bowls to be completely on your side during this ordeal.

Set up your next few days before you go to sleep before the convention even starts. Look through the schedule and highlight or mark things that sound interesting; mark all that apply, because you never know which pannel might be a boom or bust. If you can find something like a videogame room or something along those lines, set that for your idle time. With another person? Try and cooperate on some events; you never know, you might just be dragged into something you end up liking.

Point 2: Know your limits
I know that not everyone can do the marathon thing, so do not try it unless you have complete confidence in your skills. You should ackowladge any moment when you are hungry, tired, bored, or low on any personal level, as a chance to re-evaluate how you are spending your time and what you should do. You have emotional limits, and at the point where you seek to disengauge yourself from your emotions or the people around you, that might be your curtain call.

WATER. Water water water water! People do not end up drinking enough water! Food is fine as long as you eat three square meals a day (entrée, side, and helthy item), but water can be very tricky. More times than not, people will not realize they are dehydrated, and dehydration can lead to a slower response time (both physically and mentally) among other things. I suggest that at any moment, emphasis on MOMENT, you are not busy you should try to get some water. You will be burning a lot of H2O out there, so make sure to fill up!

Point 3: Etiquette!!!
This is super important. I'm sure you don't want to be remembered as "that one idiot" or "that one [word] of a person", so please listen up. First off is normal encounters; you want to make sure you smile if your talking to someone (though this can be excused since people know you might be tired) and remember your "please" and "thank you" (especially with con staff). Walking is important in the con, and how you walk is important too. Take it slow, observe your surroundings, and try not to speed past someone before they cut you off. If you can spare it, stop and wait for them to go through. Please remember to hold doors open for people going in and out of the con (if there's a steady flow behind you, this can be excused).
Even if you are cosplaying or not, this is also important! If someone wants to take a picture with you (costume or not), please don't snub them off; this also goes for random people coming up to you and spontaniously talking to you. Who knows, maybe you guys have the same interests? Also, PLEASE, for the love of all that is good, do not get angry or act like a [word] if someone gets your costume mixed up for another character or they mispronounce their name. I've had it happen plenty of times with me to the point that I don't much care anymore. Just smile, shake your head, and kindly tell them that they are mistaken; if they persist, just agree with them (don't sound condecending by changing your tone in the slightest, they will hear you, and you will catch grief for it).
In addition to what I said before, the dealer's room and artist's alley are special cases. If you meet a friend in either, go off to the side away from any booths and talk there; do not stop in the middle of the aisles, between, or in front of a booth. It is not only obstructive to everyone around you, but also inconsiderate and a safety hazard (if the numbers get large enough). Window shopping can be fun, and I have not tried to haggle before (except that one time I got pretty much blown off), but try to keep things interesting. If you see someone looking at things from the same show/game that you are looking for, try and start up a conversation. You never know, I've had many great ones in the past.

Anyway, that is my whole thing about conventions! Sorry if it got kind of long, but I tried to cover everything you might need to know. Just remember that experience is the best teacher, and be ready for anything.
Oh, and the most important thing of it all: Have Fun!!