Here's how it works. Your guardian angel hears the prayers in your heart and the ones on your lips. When you pray to God, it’s understood that God is watching over you too, and He is listening to your prayers. Here's where the angels come in. God decides upon the best answer to your prayer and gives direction to your angel to "take care of it." Think of it like this: The angels are the worker bees to God's queen bee. That's why separate prayers to your Guardian angel aren't necessary. But thanks are always welcome! 

Psalm 91:11