Things that wane simply grow smaller. "My initial enthusiasm for helping waned when I saw the massive pile of envelopes that needed sealing."
Things that wax and wane, like the moon, grow larger and smaller. Wax is the opposite of wane. A fad, or a fashion, or even a disease, that is on its way out the door is "on the wane." If your influence over your little brother wanes, he’ll probably stop copying you. Anything that wanes, like the moon, influence, or a feeling, has started to go away.