There are several things to try, from nearly zero cost up to hundreds of dollars.
A simple tub of water inside an upright piano is often enough to take the edge off the dryness in winter. Place this in the bottom of the piano out of the way of pedals and check it once a week. Margarine tubs or ice cream containers work well. Some people report good results by standing a sponge in the water to act as a wick.
If your furnace has a humidifer use it and monitor the effects with a hygrometer, adjusting as necessary.
A small room humidifier can be effective in some rooms. These cost anywhere from $50-200.
Install a Piano Life Saver System, also known as a Dampp Chaser. These are fairly costly ($575-675) but are well worth if for higher quality instruments. This is an excellent product and highly recommended. I am certified by the manufacturer to do high quality installataions. Click www.pianolifesaver.com
to learn more.