Those who didn't live through the 1970s like to believe that everyone wore white polyester suits and high heeled boots all the time. The decade was a lot more diverse than that, but these ridiculous clothing items are still fun to wear as costumes. Remembering the incredible clothing colours and styles of the 70s is important; after all, those who forget the past are doomed to repeat it.
The best way to start building your 70s look is to buy everything one size too small. The 1970s was about colour, big hair and disco, but more than anything it was about tight clothes. T-shirts should cling to your body, and if your jeans are easy to zip they're too big.
The more casual aspects of 70s clothing tend to be forgotten by the retro fashion houses. If you really want to recreate a realistic 70s look, find a blue and white striped rugby shirt with a white collar and a pair of Levi's jeans or white painter trousers. This was the way real young people looked in 1978. Top off the look with feathered hair and white sneakers.
Fashions changed substantially during the 70s. In the early part of the decade, women wore pantsuits in bright colours and men tended toward black horn rimmed glasses, turtlenecks and medallions. Fashion before 1975 was still getting over the 1960s; after 1975 it was anticipating the glitz of the 1980s. You will need to decide what aspect of the 1970s you want to recreate when designing your costume for the 70s party.
Disco is of course what is remembered about the 1970s, mostly because of its excessive and ridiculous look. If this is the look you are seeking, cruise the thrift stores in your area keeping an eye out for shoes with huge heels. Also try to find high-waisted, brightly coloured trousers made of polyester. If you're lucky you will score a classic of the 1970s: a silk button down shirt with pictures or landscapes silkscreened onto it. Wear this unbuttoned down to the stomach and put on numerous gaudy gold chains with medallions.
Gaudy jewellery was big (literally) in the 70s, and should be a part of any 70s outfit. Try to find things with turquoise in them: big rings, brooches and medallions. A mood ring or two would help too. Dangly earrings with feathers, geometric constructions or large hoops will complete your ensemble.