When you have a sauce that does not cling well to pasta, toss the pasta with a dry grated cheese, such as parmesan, before adding the sauce. The cheese sticks to the pasta and allows the sauce to cling to the pasta better.
Heat Pasta and Sauce Together: Combine the warm pasta with sauce that is warm in a heated pan. With the pan over low heat, stir and toss the pasta and sauce until the pasta is well coated. This will keep the pasta warm, infuse the sauce's flavor in the pasta and keep the pasta from sticking together. Limit the amount of time the pasta is returned to the heat source to 1 minute or less, otherwise it may become over-cooked. Serve immediately.
Toss Pasta and Sauce in a Warm Bowl: After draining, add the pasta to a warmed serving bowl and then add the sauce. Toss the pasta and sauce together until evenly coated. Serve immediately.
Serve Pasta and Sauce in Separate Warm Bowls: Drain pasta leaving some cooking water on it to help prevent sticking together. Add the pasta to a warmed serving bowl, with the sauce added to another warmed serving bowl and serve separately. The pasta should be served immediately, if not going to be served immediately, drain the pasta completely, put it back in the pan it was cooked in, and add butter or oil to keep it from sticking together.