My bedroom is tiny, and I can't even refer to it as the 'box room' as it is such weird shape and size which means that it's a killer to find furniture that fits! Despite this, I absolutely love it now that I've found perfect pieces of furniture to fit and has tried to maximise the space as much as possible.
1. Opt for light colours. I chose to paint my room white so that it looks a lot more light and airy than it actually is, but if your not into plain white walls you could choose one wall to either paint or wall paper so that you've got a pop of colour. Also, with white or cream, if you change your mind later on, on the style of your room, you can just change things like bedding or pillows rather than having to repaint.
2. Under bed storage is really helpful if you have a small room. You can by draws or boxes from places like Ikea that will fit underneath you bed and will fit all if the things that you don't reach for everyday but still need from time to time. As well as under bed storage, you could also do the same for on top or underneath your wardrobes depending on how much space you have.
3. Shelves. Shelves can look really great as well as giving you extra space in your room. I'm the kind of person who has a lot of books, magazines and films so need somewhere off the floor where I can store them. To make them look really cool you could buy some decorative book ends to go either end of the shelves.