Well, first, why is it sad? is it a direct result of something that happened in the past? If you confront the past, acknowledge that it is in the past and there is nothing you can do about it, and you are able to confront it head on (maybe you have to forgive someone, maybe you have to express yourself truly to thine own self, maybe it has to be discussed and analyzed). Once the unsaid is exposed, and the sadness addressed and acknowledged, then you can free your future of that space and be happy in it once again.
I like what Wes4617 said.
I would add. Get rid of things in it that do not serve a purpose. Paint and decorate it with bright colors and ob