Ketivah V’Chatima Tovah L’Shana Tova U’Metukah. [The Lubavitcher Rebbe included these wishes at the end of his letters. From Rosh Chodesh onwards, he would write only “Ketivah V’Chatima Tovah”, and would begin adding “L’Shana Tova U’Metukah” from around the 18th of Elul onwards.]
Although the basic requirement is to check Mezuzot and Tefillin twice every seven years, some have the custom of checking them every Elul.