It's easy to thicken a sauce by adding a roux or, at the last minute, butter, but neither technique adds that much flavor. Cooking down does! And it can turn a stew into meat with a sauce.

How to do it? Just boil away, uncovered! As the sauce boils, water will evaporate but (flavorful) solids will remain. The faster the boil, the faster the evaporation. If boiling at a high heat, make sure to stir so that nothing is burnt.

There's almost no downside to cooking down a sauce. Even if thickening otherwise later, you'll still concentrate the flavor of the sauce by cooking it down. Of course, taste periodically to make sure you're not concentrating the flavor too much; you can turn a subtle taste into something that hits you over the head if you cook the sauce down too far. This, fortunately, is hard to do.