I can't draw well at all and I tend to rely on the City of Heroes character creator interface inside the video game to muck about with costume ideas; however there are some online creators that are free and fun. I like the UGO hero machine but it actually doesn't have the one thing I'm looking for which is a basic bodice like Wonder Woman.
http://www.girlsgames24.com/game/female-superhero-creator.html
http://www.girlsgogames.com/game/Male-Superhero-Creator.html
http://marvelkids.marvel.com/create_your_own_superhero