What did Saint Michael the Archangel do to become a Saint?

St. Michael was created by God as an angel and a saint long before the universe even existed.

Roman Catholic Answer

St. Michael, being an angel and in heaven, is accorded the appellation "saint" just for that. In the 10th book of Daniel, he assured Daniel of God's protection against the Persians, in Jude 9 he is in contention with Satan, and in Revelation 12 (7-9) he drives Satan and his angels out of heaven.

Angels were created as spiritual beings. Since they have no parts or physical attributes they are each entirely unique and thus each is its own species. With angels, they have intelligence and will. God revealed Himself to the angels and the angels either choose to accept that revelation or reject it. Being angels, their choice was done with full knowledge and will, thus it was a permanent choice. Those angels that choose to receive God entered into his grace and thus heaven. Michael was one of these angels. The Church recognizes all heavenly angels as saints because their wills are forever oriented towards God and they are in possession of the Beatific Vision.
St. Michael was created by God as an angel and a saint long before the universe even existed.
The word "saint" derives from the Latin sanctus meaning "holy".
The Roman Catholic church uses it as a title for specific individuals that they believe, based upon accounts of their lives, deaths, and things attributed to their intercession after death, to be resident in Heaven. In a somewhat looser sense it means "anyone in Heaven" (it's important to note that the Roman Catholic church does not claim to "make" people saints, it merely officially recognizes them as already being saints) or even in some cases "anyone who is a follower of Christ and who therefore will eventually dwell in Heaven."

Some branches of Christianity regard Michael as being something other than a "normal" angel (examples range from "Adam, after he died" to "Christ, before he became incarnate").

However, nearly all branches of Christianity are agreed that since Michael is specifically named in the Revelation of St. John as being responsible for the defeat of "the dragon", he is unquestionably a powerful and devoted dweller in Heaven and therefore qualifies for the term.
Michael was created by God as an archangel, a pure spirit, long before the universe even existed. He was created as a "saint." Saint is a title we confer on anyone in Heaven.