Literally any liquid will do, but olive oil is typical. Adding water before you reheat works too in a pinch.

I usually find that upon reheating it stops sticking so much? Ive not really had much issue with that unless I'm trying to eat it cold, which isnt very enjoyable anyway.

Save some pasta water and heat it in a pan with the noodles and they should separate. Add sauce and reheat the whole thing in the pan.