How to Finish Pasta
Once you have made a beautiful pasta sauce, it's not enough to just ladle some onto your noodles. The Italians have a secret way of adding sauce to pasta so that the flavors really blend and complement each other.
Before draining the pasta, reserve about ¼ cup of the pasta cooking liquid. This contains a lot of starches as well as some salt (you do salt the pasta cooking water, right?)
Drain the pasta and return it to the pan over high heat. Add the reserved cooking liquid and several ladles of the sauce. Also add just a tablespoon or so of fat--either butter, olive oil, finely grated hard cheese or some heavy cream, depending on what kind of sauce you have made.
Stir very well over high heat until the sauce has thickened and coated all the pasta.
Serve on warmed plates, and ladle onto the dressed pasta as much of the reserved sauce as you'd like.
Since the water contains starch, it will help to thicken the sauce, allowing it to cling each individual noodle. As you stir in the additional fat and starchy water, you are creating an emulsion, and your sauce will develop a silkiness not possible without this step.