This is going to seem cliche…but the mantra to chant to increase the energy of the seventh chakra is chanting “Om”. (As with all of this mantra you should make this into three sounds, with the O first coming from the back the throat and then from the front mouth, followed by the m sound from the lips).
Sitting in the lotus position hold your hands out, finger locked, thumbs one on top of the other. Extend your pinkies out and have them touch and then chant “Om.” While doing this envision not just the amethyst colored chakra at the top of your head…but if you can all seven chakras over the whole length of your spine. If you can envision all of them spinning and bright while chanting it should help to improve the balance and energy of all of them.