Hey Sara, This what I use, 1 large bag of Miracle Grow Potting mix, half a bag of Cow Manure, half bag of Mushroom compost, half a bag of Nature's Helper, 1 bucket of course perlite, and 2 cups of pelletize lime. Over time the MG will become compact but the Nature's helper and perlite loosen it some and keep the medium light.
Everybody has their own recipe and this one is mine. Enjoy, cheers,