First, get the ratios right
Rach recommends cooking 1 lb. of pasta in at least 6 quarts of salted boiling water (it should taste like sea water). The salt flavors the water, and in turn your pasta, and also helps prevent the noodles from sticking together.
Mix it into your sauce
Don't dump the water after draining your penne. The pasta releases some of its starch as it cooks, and that salty, starchy water makes a great thickener for sauces. While the pasta is boiling, heat your sauce in a separate pot. Add the drained pasta and toss. Add cooking water by the 1/4 cupful, tossing until you get the creamy texture you're looking for.
Save some for later
Just because the pasta’s gone doesn’t mean the cooking is over. Freeze leftover pasta water in ice cube trays. Use the cubes to add body to brothy soups or in pan sauces instead of stock.
Read now: How To Make Pasta Sauce