No particular order.
1. Hide money in various places in your luggage. As you spend it you'll frantically search for more and when you find a dollar you'll run to the vendors and spend it, you won't get back to the room unitl later so the rest of the cash will be safe.
2. Bring at least one extra outfit. This is socks, underwear, shirt and shorts, and even shoes if you can afford it. Not costumes, clothes. You never know when something will happen and you'll have shoes full of 70 degree mocchi.
3. Build a toilet kit and put it in a ziplock bag. Those of you that haven't traveled may not even know what a toilet kit is. It's basically everything you need in your bathroom in the morning. Toothbrush and paste, deoderant, razors (which is why they're sometimes called shaving kits), some bandaids, some other pills you may need like pepto bismol or tylenol. It's great to not have to wander around trying to find a convenience store when you have a headache from Vash's elbow on the dance floor.
4. A small pen or sharpie. You will hear things, or meet people that you need to remember. Sure there's a section to write on in the booklet but what will you write with?
5. Pockets or purses. In costume or not make sure you have a way to carry stuff without bags. It's a huge benefit to have your hands free as much as possible. That way you can easily pick things up without stopping traffic.
6. A friend. As someone who travelled by greyhound bus to attend 4 days of the San Diego Comic-Con by himself, trust me on this. I'm not saying stay home, but if at all possible to drag a friend to even one day of the convention, do it. We dragged two of my friends to Saturday of Kumoricon this year and they stayed the rest of the weekend. Sharing the con is great and if you do different things you get to swap stories right away!
7. Your own snacks. I was tempted to break this down into snacks and sodas but I re-wrote the next one for that. Having a couple of Costco sized boxes back in the room as a stockpile can save you lots of cash and allow you to eat without having to walk anywhere (save on time as well as foot stress!).
8. A waterbottle. Even if you don't drink much water ever, take that bottle of Pepsi you just finished and fill it up in the sink. It's better for you than another 1.75 bottle from the vending machine and it won't stain anyone's costume. Also hydrated congoers mean less problems for staff. Fainting sucks.
9. Clothes that allow you to sit on the floor. Sure, if you spent 200 dollars on a silk kimono you don't want to get it dirty, but there's going to be places at the convention where there will be standing room only, or no chairs, or a line for an hour in a hallway. Being able to go to the floor and get back up without a ladder is a huge advantage in mobility and speed.
10. ID from the DMV. You'll probably need identification to get in, but it's something you should have. Even if you're in Middle School and have a student ID, go get an ID card made. They do that you know, nothing to do with cars. It gives out a lot more information ina pinch than a student ID, and if you're a baby-faced senior in High School whose badge says Vash and ID says Steve, it can back up the fact that you should be let into the hentai panel.
So there you go. Stuff you might not have thought of.