We brought an old fireplace back to life with a little elbow grease, a couple coats of paint and new mantel (materials found at a salvage shop). It was cheap, easy and the results are stunning.
Measure the current mantel top and plan a design. We got lucky and found a simple mantel at a recycle center that will slip over our existing mantel. We added molding to the newly found mantel to give it more interest. We attached the molding with wood screws, covered the holes, then sanded and painted the entire mantel.
Place the mantel over the top of the old mantel. Find the studs in the wall, they should be just at the edge of the fireplace. Toe-nail the screws into the mantel through the wall.
Fill the screw holes with wood filler, let dry, sand and then paint to match the mantel.