It's probably a little late to read some of this but I hope it's helpful all the same :D most of this I had to learn from experience...the long and hard way lol.
1. Always start your projects early.
This is one of the more important ones. Yes, I know that sometimes you get fantastic ideas a bit too late but are adamant on getting it done (guilty!) but starting early gives you time to test out your stuff and get adequate rest and practice. You'll thank me later.
2. Try to get enough sleep the night before.
The blind rush to get your costume and props done tends to leech your sleep and push you till the last possible moment. Yes, you finished your stuff in time for your event!
But you are tired, grouchy and have the worst eyebags ever.
No amount of makeup can hide eyebags and trust me, it's hard to have fun and/or really stay in character when all you can think about is a quiet corner to pass out in.
3. Clean up where you're able!
Scrub up, shave, moisturise, pluck your eyebrows, clean behind your ears if you have to! Treat it like getting ready for prom, you want to look your best!
Give your hands some attention as well, especially if you've been propmaking and they're stained with paint.
4. A little makeup never hurts.
Yep, this applies to the guys too!
A little bit of face powder can make a whole ton of difference in photos.
You don't have to go all out either (unless your character has obvious makeup) but cons tend to be sweaty and faces tend to get rather shiny.
A bit of strategic eyeliner also does wonders if your eyes aren't dramatic enough (too big, too small, not alien enough etc) and eyeliner makes for decent temporary face/body tattoos and even fake facial hair :D
PLEASE DON'T USE A SHARPIE I BEG YOU. I've seen cosplayers do this and not only is the shit toxic, it's hard to remove later x_x
In the excitement of a con it's easy to forget to eat; or at very worse forgetting to drink. It's very important to have SOME hydration in you at the very least. Given the stressful conditions of a con (heat, overexertion, energy sucking crowds etc) you can be in loads of trouble if you're running on an empty tank. Have a good breakfast before you go at the very least!
It can also be hard to get ahold of food depending on your con location so make sure you have some snacks/water on hand or stashed with someone who can hang on to that stuff for you. Your saikang is your best friend and rescuer.
6. Don't let people play with your props if they're fragile.
Props are flimsy. People aren't always aware of this. Also, some people get excited and crazy when you put your lovingly crafted props in their hands and they do stupid things with them.
This goes out especially to props that are long or have sharp edges. I can't even count the amount of times I've been scratched and smacked in the face by sharp shoulder plates/wings/large-stuff-on-your-back and tripped by spears/otherlongthings.
8. BE MINDFUL OF YOUR STUFF!!
In regards to your personal belongings this one.
A con is pretty much a public event. There will be thieves, pickpockets and other opportunists.
So don't be careless, granted there aren't always good places to keep your things (esp if you have a lot of junk. It's not fun :( ) but I think that when I find a PSP sitting next to a wall in a con unguarded it's already putting too much trust in the people around you =\
Check in advance if your con has a storage area. If it doesn't, try to go as light as possible or bung your stuff with someone you can trust. Lots of stuff go missing during cons and the people who find them don't always send them to lost and found.
That's all I got for now. I'll likely make another posting when I can think of more things to add in XD till then, stay safe, stay awesome!