I'll explain this as best I can in a nutshell and provide a suggested reading list if you're so inclined. Feel free to pick and choose from what i offer and disregard all or part as you see fit. I don't claim to have the only answer to this question.
A. Magic works because it utilizes natural energies inherent in plants, minerals, and other natural curios. From the dawn of time people have ascribed metaphysical and spiritual characteristics to items in the natural world.
Folks who are gifted in this regard can sense these powers, feel how they combine together harmoniously, and can tap into this natural energy to use and gain from it. An example is carrying a lucky buckeye nut or alligator's foot for good luck or wearing a piece of rose quartz to attract love and friendship. Read "Hoodoo Herb and Root Magic," by catherine yronwode for a fantastic field guide of sorts into magical herbalism in the hoodoo tradition.
B. Magic works through the intent and energywork of the practitioner. "Thoughts held in mind produce after their kind."
You've read The Secret; right; and you're familiar with The Law of Attraction? This magical principle has been majorly popularized and simplified by the aforementioned book and film, but it is a long-standing phenomenon. Your thoughts create your experience, and I'm sure you've seen this one in action.
If all you ever talk about is your bad luck, you worry and obsess about it, what are you affirming and reinforcing? More bad luck. How about your successful friend... the one who spends money freely, writes checks with abandon, feeling secure that all his financial needs are taken care of and he has plenty to meet his needs. What happens to him? He is blessed with a constant influx of money and success.
Some magic practitioners tap into this spiritual principle one way or another for themselves and for their clients. Read "The Secret," by Rhonda Byrne, "The Four Spiritual Laws of Prosperity" by Edwene Gaines, and "Creative Visualization," by Shakti Gawain. Now, positive thinking and visualization practices alone do not account for magical results; they are just one component.
C. Magic works because it aligns Diety's will with your will (or insert whatever diety or force you choose, or not). God/dess is omniscient, already is aware of the depths of your suffering, already knows what your heart desires, and is awaiting with open arms to receive you and provide everything you desire. It is just your mind, your thoughts, your ego, your behaviors, and your words which prevent you from manifesting your dreams. Magic, as well as prayer and meditation, helps to bridge that gap - to allow you to "catch up" with God/dess.
Sometimes a magic spell is, at its root, a dressed up prayer with powerful props. I'm no expert in this phenomenon, but through sincere prayer, I often get strong Divine guidance which is of great assistance to my magical work. It is God doing the work through my body, my hands, and my voice - really as it is at all times - we're just not always tuned into it.
A great book on the subject experiencing a oneness with God is "Divine Revelation" by Susan Shumsky. Another book of hers, "Miracle Prayer," teaches the technique of affirmative prayer which really shaped my prayer practice as well as my magical practice.
Any way, in my opinion magic is effected by a combination of those three powers enumerated above - amongst other powers and reasons beyond my comprehension, too subtle or complicated to mention here, and/or certainly there's powers that I have overlooked. As good book which provides an overview of magical practices is entitled "Practice of Magic"by Draja Mickaharic. Actually all his books are good.
Copyright: Me, LdyGry, A Higher Source Spiritual Supply