Sage is a perennial, which means it will grow back year after year; each year bigger and better than before. My 6-7 year old sage plant has gone through two transplants and from a 3" pot has grown to be four feet across. In early-mid summer, it has pretty blue-lavender blossoms. When winter comes, the sage leaves freeze and dry up. Even in the winter then, you can still be picking sage.
Fresh sage leaves can be cut or chopped into small pieces before adding to soup, sauces, etc.
Like many herbs, the easiest way to preserve sage is to dry it. You can do this by hanging small bundles of it in a cool, dry place. Don't make the bundles too big, otherwise the air can't circulate and inside of the bundle will mold. Nowadays I dry all of my herbs spread out on a cookie sheet in my gas oven. The pilot light is warm enough, so I don't even have to turn the oven on.
After drying, you can also make a sage powder by grinding the dry leaves. I do this using a Vitamix dry container. You could use any kind of herb or spice grinder. I prefer using powder to whole leaves, because the leaves tend to be rather large.