HOW TO REDUCE BALSAMIC VINEGAR
In small saute pan, add 1 cup balsamic vinegar or just enough to cover the bottom. Bring up to simmer over medium high heat. As soon as it begins to bubble around the perimeter, watch until the bubbles get to the center. Remove from heat and it will thicken as it cools.
Why do you want to reduce really good (expensive) balsamic? For special times when you want the balsamic to stick where you are placing it. For example when you serve very delicious strawberries for dessert. You want the balsamic be on the fruit not the dish.