I've been to every Kcon as well as many other cons and I've had a ton of issues pop up so I'm making this post so you all can learn from my experience! Herein we will discuss packing. Everything you need to have a great time at the convention without fumbling around the suitcase for things that aren't there!
First off, write down everything you're packing. Everything. And when you're ready to leave the hotel, go through your suitcase again and make a checkmark by everything on your list. That way you know both what you're bringing and taking so that you don't leave anything at home *or* the hotel.
--YOUR COSTUME. Examine the character. Examine the character several times. Write down every single thing they have on them, including the things you're not including in your costume. Make notes of what, specifically, you're leaving out. Pack everything else. Examine the character again. Repeat until you're 100% certain that everything is in your suitcase. Then check again. Once I forgot my big foam tongue to my Quina Quen costume and that's the primary prop.
--FIVE PAIRS OF UNDERWEAR. Even if you don't change it every day, you should be changing it at least once during the con I mean come on. The rest are backup in case of unforeseen accidents, stains, holes, or even if you spill something and need to grab something to mop it up.
--FIVE PAIRS OF SOCKS. Socks are like sweat magnets and no one wants to be That Congoer. No matter how much you shower, it's still going to be in around the 70s at the con and there's not a thing personal cleanliness can do to stop that.
--MAKEUP. This is primarily linked to cosplay but includes many other things as well. If you need something to cover acne, bring it. If you need lip balm, bring it.
--MEDICINE. Similar to above but even more vital, especially if you're coming from far away. Allergy medicine is going to be a biggie this time of year and you don't want to go the weekend without it. And DO NOT stop taking your medicine just because you're at the con--even if it's unintentional, skipping is something you don't want to do. Bring your medicine and MAKE SURE you take it. Put it by the sink. Put a reminder on the mirror. If you have to take it during the day, set your watch or phone or pocket pet to alert you of the time.
--CLOTHES. Normal clothes. Even if you drive in and back in your costume, you want to have at least a set of everyday clothes. Say you need to pop off to the grocery store or something, or you decide to go chill in Portland for a few hours, or even you've taken your costume off for the day and decide that you want to head down to the lobby to hang out and don't want to put the whole thing back on. Related to this is
--NIGHTCLOTHES. Holy crap I can't even count the people who've forgotten nightclothes and had to sleep in underwear. Bring at least a tshirt.
--TOOTHBRUSH AND TOOTHPASTE. Yes some hotels provide toothpaste. Most don't. And you don't want to have to leave the con to buy these things, because you WILL need them. Again, you don't want to be That Congoer.
--RAZOR. Most hotels will provide these, but if you're rooming with anybody, you'll want to bring your own.
--CHARGE CORDS. Your phone, your games, your laptops, and anything else you're bringing with you. Even if you leave your phone in your room and only check it once a day to see if your parents called, you still bring the charge cord. Even if you think your 3DS will be fine because you're only taking it for Street Pass, bring the charge cord.
--BATTERIES. You WILL run out of batteries for your camera. That much is a fact of life. Bring at LEAST one change for it.
I'm sure others can add onto this!