Specifically a coin? You can dedicate any item to aid you in magick by casting a circle, placing your hand over the object and asking the God and Goddess to bless the item to be used in your magick. But if your looking for more power in your spells look within. Write your own spells so they are your exact intention and energy, use materials and ingredients that a personal and purposeful. Study associations and practice focusing your intent. These are the basics to becoming a powerful witch with successful spells.