How to Finish Pasta
Once you have made a beautiful pasta sauce, it's not enough to ladle some onto your noodles. The Italians have a secret way of adding sauce to pasta so that the flavors blend and complement each other.
Before draining the pasta, reserve about ¼ cup of the pasta cooking liquid. This contains many starches and 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--butter, olive oil, finely grated hard cheese, or some heavy cream, depending on what kind of sauce you have made.
Stir well over high heat until the sauce has thickened and coated all the pasta.
Serve on warmed plates, and spoon onto the dressed pasta as much of the reserved sauce as you'd like.
Since the water contains starch, it will help thicken the sauce, allowing it to cling to each noodle. As you stir in the additional fat and starchy water, you create an emulsion, and your sauce will develop a silkiness without this step.