The problem with turning an attic or other upstairs room into a bonus room is that the slope of the roof often cuts into the space. While you can't do anything about that structurally -- short of rebuilding the roof -- you can use paint, light and furnishings to make the room look bigger. More than that, you can use the sloped walls to your advantage and actually create more living area.
When it comes to the walls and ceiling of a room, the eye interprets lighter colors as more spacious, and this effect is even more pronounced when the color scheme is monochromatic. If you use the same color paint for the walls and ceiling, however, the ceiling often appears darker because of the different angle of viewing. This lowers the ceiling visually and cancels out the benefits of the light color. To avoid this effect, paint the ceiling a lighter shade of the wall color. Use two adjacent colors from the same color chip to ensure a good match.
Any room looks larger when it's well-lit. Maximize the light that comes through the windows by using sheer curtains or no curtains at all. Install a skylight, either in the ceiling or in the upper section of one of the sloped walls, if it is part of the roof. At night, direct lighting is your friend. You need ample illumination at ground level, supplied by floor lamps and track lighting placed low on the sloped walls. Leave the sloped walls dark, but shine one or two of the track lights on the ceiling to draw the eye away from the slopes and make the room seem larger.
Not only will the room appear larger, but it will have more actual space if it isn't full of clutter. The sloped walls offer a natural solution: Turn the unusable space where the walls meet the floor into storage space by building vertical walls and framing cabinet doors into them. Not only will you have lots of extra space for your stuff, but you won't need to light the lower portions of the walls that make the room feel closed in when they're dark. The walls don't need to be high -- 24 to 30 inches should give you lots of cabinet space.
One common-sense guideline regarding furnishings is to avoid having a lot of them. Keep enough furniture in the room to make it utilitarian and comfortable, but no more, and don't cover the walls with colorful objects. Bring mirrors into the room instead of cozy wall hangings. Placing at least two so that they are facing each other is an effective technique to create the illusion of space. Avoid attaching the mirrors to the sloped walls, however, or all you'll see will be the floor projected on the walls. If the room doesn't have two vertical walls facing each other, place at least one large mirror so that it's facing outside.