There are some things that without experience you just do not think about – and even then with experience you sometimes still forget! Here's a list of items I have often forgotten over the years when packing for cons. Some are rather minor, but others can cause some difficulty and annoyance if missed.
Enough Money for the Weekend
The thing about ATMs is, they're only restocked on banking days. This means holidays and weekends, which are what most convention days are, if the ATM runs out, it's out until the next business day. For this reason it is very important to plan out how much cash you are going to need to get you through the whole weekend. If you're driving, don't forget to factor tolls into that, and be sure to check if/how parking in the area needs to be paid for. Also think about food. Unless you know for a fact a place takes credit cards, plan to be buying meals with cash. Same goes for anything you might want to buy in the dealer's room. Finally, make sure to arrange how the hotel payments will be squared up: if it's during the weekend and you owe money, set that exact amount aside in an envelope so you don't end up accidentally stiffing your roommates.
For a lot of us, the big focus of attending conventions is to cosplay. Realistically, though, you might not want (or be able) to spend the whole weekend in costume. What if your friends want to go off site for dinner and all your costumes are a bit too outlandish for public? Or what if something breaks/rips, or just is too uncomfortable for such long term wear? Or, simply, what if you just don't feel like wearing a costume (I know, perish the thought)?
The point is, there have been cons I've been to where I cause myself problems by underpacking on “civvies” or non-costume clothes. Another big thing I need to impress upon you is, whether costumes or not, PLEASE don't wear just one same outfit all weekend. This is a major way to contribute to “con funk” even if you are showering (and you better be showering). If you just have one costume, this means taking breaks and wearing civvies. Alternately, bring a couple costumes and alternate throughout the weekend.
Appropriate, Comfortable Footwear
Now, obviously this isn't always possible when you are cosplaying (Though I caution you to try and find/make cosplay shoes that don't eat up your feet if possible), but when it comes to walking around out of costume do not forget footwear! Trust me, I tried to get by at Dragon*Con doubling my cutesy flats from Coco Berry for my out-of-costume shoes and gave myself quite the blisters walking around. Of course, for an event as spread out as D*C this is more of a concern, but even if you don't have a far distance to go you may end up walking farther than you think just going back and forth through the convention halls.
If you're going to be in costume, you'll want to have make-up on. But what happens at the end of the day when you're ready to go to bed? Depending on your skin type, leaving make-up on can cause acne flair ups. Even if your skin isn't so delicate, left-on eye liner and mascara is notorious for smudging overnight and without make-up remover you can rub the sensitive skin under your eyes nearly raw just to get it off, let alone the risk of getting stinging soap in your eyes!
Furthermore, if you're using more heavy-duty make-up like body paint, you should seriously consider how you're going to get that off, because if too much is left on your hotel towels you might incur a charge for extra care in laundering them. Depending on how much body paint you use, make-up wipes can work. Or, perhaps you want to simply bring your own wash cloth and towel.
Emergency Repair Kit
You never know when something will need to be fixed! See my previous entry on the usefulness of and how to put together an Emergency Repair Kit.
You really should be showering at LEAST every other day of the convention, more if you sweat a lot or wear heavy-duty make-up. Therefore this is not so much a reminder as a desperate plea: Please bring AND USE toothpaste/toothbrush, soap, and deodorant at the very least. You'll probably want more (though the hotel should supply soap, shampoo, and conditioner) but specifically what is up to you.
Medicine/First Aid Kit
If you take prescription medicine on any sort of regular basis, this should go without saying, but if not it doesn't hurt to bring some aspirin and a few band-aids as well just in case you need them.
Final Tip: Make A List
If you're at all like me, making a list of things to do or to pack before a con may have a very calming effect. I might tend to go a little overboard, listing to the most precise details of what I need, but it does help me avoid that nagging feeling in the car I may have forgotten something, or worse getting all the way there and realizing I did. Of course, then the battle becomes remembering to list everything, but that's where the above suggestions come in.
Think I forgot to list something? PM me on facebook, or email me and I may edit your suggestion into this post!