add a mirror to the room you are thinking about. Mirrors are a great way of making a room feel bigger but also they reflect the light which will bounce into every corner of the room.
Make sure you have chosen the right colour palette. Colour can either absorb or reflect light. Light bright colours are key as they also act as a mirror.
Also consider what window dressings you have. Long heavy fabric curtains can darken a room whereas roller shades only take up the space within the window frame and can be pulled up during the day.