How to make my new shared bedroom look bigger?

I'm moving into a new house, and my sister and I are going to share a room. It's the master bedroom so it has a bathroom and a big closet, but it's not that big. And it has a low ceiling... :( We're painting it a lightish purple, and the room will have a couch in it, but how can I make it look a lot bigger? Any tips/ ideas?