1. Color schemes. A formal room is usually based on a more formal color scheme. While blues and yellows and reds are inviting, and encourage informality, golds, deep greens, and dark reds with deep bronze or purple undertones encourage a more formal atmosphere. So, choose a color scheme that while inviting, maintains a formal feel to it. Choosing turquoise, red, and yellow is not going to make the room formal, rather playful. Whereas, gold, green and rich browns make a room feel richer and more formal.
2. Choose your furniture wisely. An informal living room would have furniture that is comfortable, easy to clean, over stuffed, etc. a formal room usually has furniture with floral designs, or sleeker lines. The room tends to have lots of finery and extras. While couches may be comfortable, they ought not to be so inviting that you want to spend all of your time on them. Instead, look for items that are maybe better looking than feeling, and that give off the look and feel of formality. Cheaply made furniture does not give off the impression of formality, so get heavier pieces, items made of real wood, not particle board, or wood composite. Choose glass topped tables with heavy iron bases, brushed gold frames, floral arrangements, and candles, add in some books on philosophy, and nice artwork.
3. What isn't there is as important as what is. Most formal sitting rooms will not have any sort of technology. In other words, you do not put a television in a formal sitting room. This would encourage everyday use, and formal rooms are supposed to only be used for formal reasons. In addition, you may want to have books, and coffee table guides, etc. in the room, but magazines, and less formal reading materials would be better served in another place in your home. You usually will not find snap shots of pictures of the family in a formal sitting room, possibly one portrait of the family, enlarged, but generally artwork adorns the walls in these rooms.
4. Music. A musical instrument is often found in a formal sitting room. A piano, a harp, or some other instrument that can add to the grandeur and formality of the room. This is especially true if it is one that is not often used, such as a grand piano, or an antique. Even if someone in your home plays, it is likely they have a different instrument that they practice on.

These are just a few suggestions for taking a normal sitting room, and giving it a more formal feel.