Creativity is the name of the game
Besides the music and dancing, most of the fun in raves come from dressing up. The only fashion rule is to be as creative as you can be. A rave is meant to be a place of self-expression and creativity. But we do have three important and helpful tips to give for your own comfort.
BCBC - "Be Creative But Comfortable" - your outfit can be as out of this world as you want it but do remember to consider your comfort, whether it be with shoes or keeping warm.
Keeping on the subject of keeping warm, check the weather so you're sure to be prepared. You wouldn't want to get too cold or hot and risk having a bad time because your outfit just wasn't right for the weather.
Know that some raves or festivals have fashion restrictions - be sure to check the festival's FAQ's to make sure of any they may implement. Each rave or festival has its own image or fashion expectations. While there's no judgement in wearing whatever you please, it's always good practice to research about the event first.
Other than those, there is nothing much to say. You're only limit is your imagination.