After all, this is the fast that saved the Jews from Haman back then.
(Also, a summary of the linked article is in the comment section below.)
Giving any kind of tzedakah and reciting Psalm 22 (whether once or 7 times, as recommended by the Kav Hayashar) is an excellent thing to do throughout the fast day.
Saying a lot of Tehillim on Taanit Esther & Purim is very auspicious, if you can manage it.
The main thing on Taanit Esther is pouring out your heart and asking Hashem that He help you in the zechut of Esther Hamalkah & Mordechai Hatzaddik.
The fast starts at the crack of dawn on Wednesday morning, and you need to start the segulah from that moment.