I came across this fantastic video that actually teaches the correct way to chant the Ganesha mantra and what prasada may be given to Lord Ganapati as an offering. I think the most apparent mistake many people, especially Westerners, make in mantra yoga is incorrect pronunciation. It has been incredibly helpful to me to listen to mantras repeatedly to learn to pronounce every Sanskrit syllable correctly. The mantra cannot be spiritually energized if chanted incorrectly and some mantras I hear can actually have adverse effects upon one's person, so please take sufficient time and care in learning not just the mantra itself, but also the meaning. There is much more to be gained when we treat every Sanskrit mantra with the deepest reverence.