Upon immersing in the mikveh, one recites the brachah (blessing) "…asher kideshanu bemitzvotav vetzivanu al hatevilah, ….who has sanctified us with His commandments and commanded us on immersion." Many Sephardic women recite this bracha while wearing a robe, and then undress and immerse in the mikveh. This is in accordance with the general practice that a bracha should immediately precede the performance of a mitzvah. The prevalent Ashkenazic custom, however, is to immerse once, recite the bracha while standing in the mikveh with the water at least waist-high, and then immerse again.
Books of techinot, traditional women's prayers, are widely available at Jewish bookstores and from www.mikvah.org, and often include prayers for immersion. Some women prefer to recite the "yehi ratzon" from after the shemoneh esrei. Other women choose to recite a personal prayer. All of these are typically recited in the water, but may be recited before or after immersion, as they are a matter of custom and not obligatory.