To flirt effortlessly, you have to develop your self-confidence first of all. This is because, no matter the principles I will teach you now, without the right attitude to get it done, you might end up messing up. However, beware of the person you want to flirt with also, so it won't end in disgrace. Be sure the other partner is ready for such.
So, to flirt with someone, make sure you have good and fixed eye contact. This first sends a message to the other party that something is coming. And as you fix your eyes on your partner, make sure you add some nice smile. Also, create a sense of humor. Try to make him or her laugh, especially if you, as a man is trying to flirt with a woman.
Actually, women love funny guys. Also, buy a gift for your partner. This is also a means to really flirt with someone. Lastly, here, develop the habit of a sincere compliment. However, compliments can only spice up other flirty acts; it is not flirting on its own.