If you haven't already, there is a special prayer you can say during Mincha toward the end of Shemoneh Esrei in which you set yourself up for fasting tomorrow (but this same prayer also lets you off if you end up unable fast).
(Please see his original letter, plus an English translation HERE.)
The Gadolim of Eretz Yisrael also call upon us to say Tehillim together on 29 Adar/March 25 at 5:30 PM Eretz Yisrael-time: Psalms 13, 20, 91, 112, 120, 121, 127, 130.
This is followed by Avinu Malkeinu & Acheinu Kol Beit Yisrael.
A worldwide kabbalat ol malchut Shamayim is at 6:00 PM.
(Please see the original announcement HERE.)
(Please see here & here for more on how to do that.)