two weeks before the con(and leading up to it):
DO:
Double check with your friends about plans (who is ridding with who? what goes with who? Is every one still going? Does everyone have a way to contact each other? who made the reservations?)
This falls in line with the first point but confirm all transportation
Make sure you're clear from Work/school/family obligations/ insert anything here really.
Go over your budget, it's always better to bring a bit more than you think you'll need than less
Make sure you have what is required of you from the Con to get your badge.
Make sure you have a memory stick (or two) to dedicate to the event, or buy film, or the disposable cameras, whatever you use.
Make a kit that has everything you need to make repairs on your costume
Make a list of EVERY THING you are bring to the con. Including all of the above
Finish your costume
DON'T:
Put all of this off

Night before you leave for the con:
DO:
Check your list you made , check it twice, pretend you're Santa if it helps you.
Finish your costume (but you already did that, right :P )
sleep
DON'T:
Panic, if something has gone wrong, panicking won't fix it.

When you get to the hotel (or friends house, ect.):
DO:
Be polite at check in, these people may or may not be used to events like an anime/scifi/goth/ect. con.
Take time to organize things in your room, determine who is sleeping where.
DON'T:
Be a snob/jerk/insert insult of choice here. I don't care if the employees are being snobs/jerks/insert insults of choice here. Be the bigger person and be polite and then report it to their manager.

Day(s) of the con:
DO:
Shower
Carry around a small repair kit with everything you might need to fix your costume
Control your volume level. Yes I relive you are there to have fun, but so is everyone else. The people thirty yards down the hall don't really care most likely if character a and b get together or how cute character x looks in episode 279. You do, and it's fine to be enthusiastic, but try to control it as well.
If you have minors in your group, make sure you account for where they are and what they're doing.
Shower
Eat
Ask before you take a picture of a cosplayer, and make sure you have their consent.
Respect the constaff, same thing as with hotel employees, report them to those above them and keep enjoying your week end. A lot of the times they ask you to move, ect, because there is some kind of a safety hazard involved. Is it that bad to take three steps to the right?
Wear your con badge if required.
Shower.
Sleep
Use your head, if you think "I would never do this outside a convention," you might need to rethink what you're doing
Be helpful, if you see someone in a large costume or a costume with a large prop, get the door for them, hold the door open for people coming in and out of the buildings.
Did I mention Shower?

DON'T:
Talk smack about cosplayers
Talk smack about other Fandoms
Insert random Japanese words into your conversations just for the sake of using Japanese. It doesn't make you sound smart, it makes you sound like you don't know how to speak English or Japanese properly.
Steal from the artist in artist alley or from the dealers in the dealer's hall. I could give you a million reasons why, but I can sum it up. It makes you into a huge jerk and it's illegal.
Don't vandalize ANY THING throw your trash away, and don't bother guest that aren't part of the Convention. The hotel may not let the con back if these things happen.

After the con:
Make sure you have everything, double check the room, triple check the room.
Try not to leave the room completely trashed, yes, they pay people to clean up your mess, but try to keep it minimal.
Get some sleep when you get home, you earned it!

SAFETY
It's amazing how many people ignore simple safety rules
There is safety in numbers, but don't be unreasonable. If you don't know the area or it is knows as a 'bad' area, keep at least one friend with you. But on the opposite end you don't need twenty people to go to the restroom.
Don't just go off with random strangers you've just met, no matter how nice they are.