Decorations: As this is a party, I would steer away from the darkness of some superhero flicks and stick to a cartoonish tongue in cheek theme. Super decorations can be found mostly everywhere, try supermarkets such as Tescos and Asdas or shops such as HMV for premade decorations and superhero posters whilst shops like Poundland or Wilkinsons offer cheap solutions to create your own.
Costumes: Now this is when the fun begins. There are thousands of superheroes to choose from, which one you choose though is completely up to you. There are plenty of sites on the net to buy your own costume - but where is the fun in that? (And it is very pricey). Why not make your very own Ironman costume, that even Tony Stark would be jealous of, or even a purrfect catwoman outfit.
Want some foremost party games that both kids and adults can enjoy? Why not play pin the hammer on Thor? Or bob for green apples aka kryptonite, just don't let Superman play that one. Need music that is fit for superheroes to boogie too? Why not burn your own CD filled with superhero theme tunes or music found in the movies.
Finally, you can't let your heroes fight crime on a empty stomach can you? Why not make your own super party food. From Hulk shaped biscuits to Ironman cakes. You may also want to make your own superhero drinks!