Just have a few tips to share with you for now. However, if you have any of your own, please feel free to share and send them in so we can post them for everyone to enjoy.
Tip 1: Many of these costumes can get hot super fast. Fill a zip-lock bag with a very small amount of water (so the bag is still thin), and line parts of your costume with it. It works well in the chest and back areas, and sometimes if you keep the water minimal it can even fit in the arms.
Tip 2: Always talk in character of the costume you're wearing. Self explanatory.
Tip 3: If you're planning on being outdoors late at night and have a costume thats dark, consider taping a strip of removable caution tape (bright orange) to the back of your getup. It'll help keep you safe, and you can usually use a couple smaller strips to make a slick design and add to the costume's overall awesomeness!