I think that the most enjoyable way to decorate a den is to make it look like a wilderness type lodge. Lots of wood, super comfortable furniture, wilderness pictures. It is usually best to go with a theme and stick to it.
You want this room to be relaxing. Pick relaxing colors. Warm and cozy. Carpeting, wide screen TV, 2 comfortable chairs, couch, reading lamps, bookcase and any other items that will make it cozy. It will be great if it had a fireplace or gas burner. Pick out a theme and for it.
Begin with a warm, cozy color, even a deep one if you're not afraid of color. Bookcases, barrister style if you can afford it, are important with neatly kept books and decorative bookends and other items. Add suitable large, nicely framed posters or art to taste. Consider a large object or two, such as a globe or sculpture.
Most dens are dark by nature, if you are going to use the room for your main television viewing I would suggest that you stay with warm colors and refrain from using anything to bright. Have to couches in the same color, something other than beige, facing each other.