One good place to start is w/ primos hoochie mama call and the rest of their diaphragm calls, their video "mastering the art" is also very good to get started. Everyone I've heard from agrees that elknuts video is good, not one complaint, although I haven't personally seen it.
that primos call works wonders, once called in 3 bulls in Colorado and they came running at me literally and shot a nice 5x5 bull with my bow.