-- Blue liquid laundry detergent can be used to paint designs (stars on the ceiling, whatever) that glow under black lights -- much cheaper than glow-in-the-dark paint. Easy to clean up later too
-- Mix water-based tempera (a few dollars a bottle or cheaper) with a little bit of liquid soap, mix slowly (so it doesn't get all sudsy and bubbly), and you can paint your windows. The soap helps the paint stick to the glass and makes cleanup easy.
-- I like to leave at least a wall or so free to decorate throughout the year with programs, ticket stubs, cards, etc.; good ads from magazines and flyers are fair game too. At the end of the year, packing it all up in a folder or envelope makes an easy way to keep mementos if you're into that sort of thing.
-- Color printer + digital art. Use your own or get your money's worth out of computing fees by going down to the computer center and using the large format or color laser printers there
-- Most poster stores will have a rack or two of cheapie/sale posters for just a few bucks a piece. The time spent looking can often result in some great finds.
-- The classic empty drink bottles. Even in a "dry" dorm, nobody can say anything if your collection of Absolut or wine bottles is empty
-- Hit a dollar store or two...self-explanatory
-- This only works later in the year...but can be useful for following years: calendars go on MAJOR sales (50-75% off...free if you know people who work in bookstores) in January. Pick up one for regular use...and pick up a few with good pictures.
-- Invest in GOOD sticky stuff to keep your stuff on the walls...nothing more frustrating than having your posters peel off and fall on you when you sleep
~Ladi