A reason why we start to wish each other ״כתיבה וחתימה טובה״ on Tu B'Av is that the gematria of ״כתיבה וחתימה טובה״ is exactly the same as ״חמשה עשר באב״, because now begins the preparations for Rosh Hashonah. (Reb Chaim Elazar of Munkatch)
This fits in well with the Gemara (Bava Basra 121b, Taanis 30b-31a) that says that starting from 15 Av, when the nights start getting longer, one must add time to his Torah study at night. If one does this he will have days added onto his life, and if one doesn't, then the opposite, Chas v'Shalom. It would seem that a basic prearation for Rosh Hashona would be to add time to one's Torah study.
״כתיבה וחתימה טובה״ to all of Klal Yisroel