The word segulah might be translated as a spiritual remedy or an auspicious tradition. In the Bible, however, it is used in the phrase Am Segulah, a treasured nation (Exodus 19:5; Deuteronomy 7:6, 14:2, 26:18), and refers to the special relationship God has with the Jewish people.
Perhaps then, a segulah might be understood to be an action that demonstrates a treasured relationship with God by doing something extra. In the case of praying or reciting the Song of Songs for 40 days, this makes immediate sense.
One can certainly find an abundance of segulot. The most important purpose of the segulah is drawing closer to the G-d. The best segulah that I know is personal prayers.