The Arizal (Rabbi Yishak Luria, Egypt-Israel, 1534-1572) taught that men should immerse in a Mikveh each week on Ereb Shabbat, and this is, indeed, a most worthwhile practice to observe each and every Ereb Shabbat throughout the year. According to the Arizal, one should immerse twice: once to divest himself of the weekday "clothing" covering his soul, so-to-speak, and a second time with which one dons the spiritual Shabbat "clothing."
Another custom, however, requires immersing three times on Ereb Shabbat, particularly if one had contracted Tum’at Keri (ritual impurity resulting from semenal emissions) during the week. The first immersion serves to purify oneself from his state of Tum’a, whereas the second and third serve to remove the weekday spiritual essence and bestow the Shabbat spiritual essence, as mentioned above.
Rav Yaakov Haim Sofer (Baghdad-Israel, 1870-1939), in his Kaf Ha’haim (Orah Haim 260), records yet a third custom, to immerse thirteen times.
The Arizal had the practice not to dry himself after the second immersion, symbolizing the desire to maintain the spiritual "clothing" of Shabbat that one obtains through the second immersion. Those who follow the Arizal’s custom of immersing twice should not dry themselves after the second immersion, whereas those who immerse more than twice may dry themselves.
The Arizal would don his Shabbat clothing immediately after the immersion.
Summary: It is proper to immerse in a Mikveh every Ereb Shabbat. Some have the custom to immerse twice, others immerse three times, and yet a third custom requires immersing thirteen times. Those who immerse twice should try not to dry themselves with a towel after the second immersion.
