How to Decorate Your Fireplace

What is the rule of thumb, as to how high to hang art work over a sofa or table?

I have a long and narrow lving room. It has an opening to the hall on the opposite wall, 2 windows and a fireplace take up the whole end of the wall and the other end is an opening to the dining room. I have the long white couch against the long wall with a circular coffee tabel and love seat backed to the opening of the room near the entrance hall. If it isn't challenging enough, I have an open staircase with a angled wall near the dining room opening where I have a sofa table. It is painted blue and my throw pillows are different shades and prints of blue. I have a chair backing the dining room with a blue paisley print. Its a beautiful room but I have always wanted it to be a little more contemporery...We cannot afford to buy new furniture. Both sofas are white damask and the chairs and tables are all light beige wood. The coffee table is glass with wood.
Do you have any suggestions? I wish I could put pictures up on this site to show you....Doris