How do you say no to someone asking you out?

Politely say "No, but thanks for asking me!"

or if you like them but not in that way say, "I'd rather just be friends"

NEw aNSwer: or if you don't wanna hurt their feelings say u have a boyfriend if they ask who just grab a guy friend and say its them! (IT WORKS OUT TRUST ME, i should kno ;) )