I'm surprised that it hasn't been mentioned yet that the Jab helps to set your range. What I mean by that is that the Jab is a visual cue letting you know that you are in range for the cross. My cross has about 6 inches further range on it than my jab, so if I'm right at my opponents face with the jab, I'm perfectly set for the cross.
Also, I've always taught (and been taught) that the power comes from the whole body - legs, hips, and shoulders. For example, it's the torque developed by turning the hips and shifting to your foreward foot that develops the power of a lead hook. It's the hips and shoulders that develop the power in the cross - along with bringing up the rear foot, of course.
One last note on hooks. The hand position of a hook varies with the target zone of the punch. The lead hook to the head is palm down, the rear hand hook to the body is done palm facing in. For a visual, place your hand flat on your belly. Make a fist and raise your arm over your head by lifting your shoulder. you will see your palm go from facing in (your belly) to facing down at head height, to facing out (away from you) above your head. This is a natural rotation and to be utilized when using a hook.