One thing that I've always enjoyed, is always keeping a backpack with survival-like items in it (Okay, messenger bags are way easier to deal with in a crowd, so that more than a backpack >.>
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A bottle of water or two, hand sanitizer, some munchies (I take canned fruit with a pop top lid), sewing kit (black needle and thread will get you through almost anything), baby powder, band-aids, scissors,a lighter, some dice, a deck of cards (playing, magic the gathering, pokemon, or other) Deodorant, Pens, Paper, and Phone Charger.
I also try to have 5-10$ in change sitting in a bag at the bottom of my pack for emergencies.
One thing I've noticed, is that since most convention venues are inside of heavy duty commercial buildings, and often times have things in the basement, and high up, cell phone batteries don't last as long. So making sure to charge your phone/have an extra charged batter with you (I bought one for a phone a few years back for $10 on ebay) can be a huge life saver.
If you're the camera type, having a spare battery/memory card/place to unload images is pretty crucial too (spare batterys for most digital cameras are fairly cheep as well. Ebay once again has most for $5-20).
I've yet to attend a con where charging something electronic wasn't an issue for me, so I've also started keeping extra phone chargers around (and this year Kumori, even an extra phone...) for emergencies just in case. Fry's electronics will sometimes have deals on pre-paid phones, where you get the phone, and enough air time for unlimited text/calling for the entire con for $10-20. (I like to think that last part is good practice. I keep a charged pre-paid phone in the glovebox of my car at all times, just in case I need it for some reason... being stranded is pretty much the worst thing ever >.>)
Pen and paper is one that is also kinda huge for me too, just because it doesn't fail. A laptop/phone/pda/ipod/(device) always has a chance to not work for some reason, and having a pen and paper is an (almost, pens do break) easy way to not have problems when trying to give your contact information to someone (cons are for making friends after all =^.^=)