What do you do with all your stuff?
For me, I always have the stuff with something the character will have. So it could be a purse or maybe a hallowed book. Luckily for me, if my character does not have any, I still have my sisters. At least one of us will have some sorta character that has something to carry stuff in. If all else fails, I would sew a hidden pocket on the inside of the cosplay but that is normally hard to get to.
How do you get good cheap food?
I never buy con food. I always walk some ways or take the bus somewhere and get food away. It is cheaper that way.
What do you do if you have an elaborate/heavy/uncomfortable costume?
Just take a break when you are tired. Feel free to even sit on the floor. =) Also, watch where you are going and where other people are going. With lotsa people, there is lotsa bumping so you have to be really careful to avoid to much collision.
As for the plane thing, hopefully you don't have anything to big. I would try to have it as a carry on since I wouldn't trust the airport baggage claim thing to take good care of your item as it drops from a height.