When pasta cools down in the refrigerator overnight, the starches congeal and get sticky. If you use a microwave to heat it up, you'll only make it stickier and dryer.
Give leftover pasta a quick dip in boiling water to dissolve some starch. This makes space in the noodles for water to be absorbed, giving the pasta the right texture.
You don't need to boil your cold pasta for very long. Drop your noodles into the pot, let them bubble for 30 to 60 seconds, drain away the water, and they'll be ready to serve.