One should decorate the body with tilaka, which is the sign of the Vaisnavas. The idea is that as soon as a person sees these marks on the body of the Vaisnava he will immediately remember Kṛṣṇa. Lord Caitanya said that a Vaisnava is he whom, when seen, reminds one of Kṛṣṇa. Therefore, it is essential that a Vaisnava mark his body with tilaka to remind others of Kṛṣṇa. In marking such tilaka, sometimes one may write Hare Kṛṣṇa on the body.
Source: A.C. Bhaktivedanta Swami Prabhupada (2011 edition), “The Nector of Devotion”, Page 54