The advice that the alchemist gives about listening to one's heart is that he encourages Santiago to listen to his heart so he will be on tune with the world and be able to follow his dreams. Okay, i think i agree with him.
According to Alchemist; God created the natural world so people could understand that a more perfect world awaits, the natural world is to be used for spiritual teachings, a single grain of sand symbolizes the whole physical world and can lead to a vast understanding.