2. As with anything take inventory of your body type. Buy a costume that best suits that. For me I know that flare skirts or A-line skirts generally flatter my wide hips. For bustier ladies you may want to rethink that strapless costume for something with a little more support.
3. Even though it is a costume you should still wear you body foundations & girdles....smooth is always better.
MSN released an article on curvy costumes that you can check out for some ideas. Here are some of fav. picks on the list. And remember you can always get a little creative with some accessories and use the clothing of your choice...which may be more flattering for you. You can also check out http://www.anytimecostumes.com/plussize.html