Illness-One who is sick or fears to immerse due to health reasons:
One who goes to fulfill the mitzvah of immersing in a Mikveh will be guarded so no evil happens to him. Accordingly, there is a tradition from the Bal Shem Tov that one should not fear any damage to ones health from a single immersion in a mikvah. Likewise, there is a tradition in Chabad, that the Rabbeim take the responsibility of a single immersion. Nonetheless, in the case that there is real worry of danger to one’s health one should not rely on this unless he truly trusts that he is deserving of this protection, or speaks with a doctor. [Certainly, if he contains a contagious illness, he is not to immerse and place others in danger of getting ill.]
One whose ears cannot get wet: One whose ears cannot get wet is to place a wet cotton ball in his ear, based on his doctor’s directives.
How to immerse:
How many times should one immerse in the Mikveh? When immersing in order to purify oneself from the state of Keri, a single immersion within the water suffices. When immersing as a symbol of Teshuvah, one is to immerse three times. There are numerous opinions mentioned in regard to additional immersions. [Practically, one is to immerse at least three times by the daily Mikveh immersion for the sake of Teshuvah. The Rebbe negated the rumor that he stated that one should immerse five times, and did not arbitrate any final number to immerse. Nonetheless, Rabbi Groner quotes the Rebbe to have directed him and others to immerse nine times, and that so is the accepted practice. The Rebbe himself was witnessed to immerse many times, very quickly, one time having counted 51-52 immersions. On one occasion, when the Rebbe was asked as to how many times he immerses, he refused to answer, and on another occasion he responded to a person who connivingly hid to count his immersions that it could be that he stuck out his finger from the water and the person did not see.]
Preparation for Mikveh-Chatzitza: When immersing for purposes of Keri, one is not required to remove a Chatzitza from the body prior to immersion. [Nevertheless, some Poskim are stringent.] Thus, one does not need to cut his nails, clean them, brush his hair or wash other parts of the body. It is likewise not customary to be particular in this by the daily immersion, aside for a general showering of the body and removal of clothing. Nonetheless, some conclude that even by the daily immersion, one should be careful to avoid all Chatzitzos. Despite the above, regarding the immersion of Erev Rosh Hashanah and Erev Yom Kippur [which is done also for the purpose of Teshuvah] one is to remove all Chatzitzas from the body. Thus, one is to clean his body with hot water, cut his nails, and comb through his head and beard hair prior to immersion, in order to remove any intervals. One is to cut the nails of both his feet and hands on two different days in preparation for this immersion. One is to also brush and floss his teeth.
The Kavanos: Although the Mikveh effects the person even if he immerses habitually without anything in mind, nevertheless, it is a great advantage for one to immerse with the proper concentration. The Mittler Rebbe writes that it does not suffice to immerse without the intent of “Bittul Ruchni,” of thinking of one’s nullification to G-d in doing so. Various other Kavanos are also recorded when immersing in a Mikveh, some based on Kaballah and some on Avoda.
Taking a shower after Mikveh: It is permitted to shower after Mikveh, although some are stringent not to do so. Nevertheless, on Erev Shabbos [and Erev Yom Tov] according to all it is better not to do so, in order so one not completely wash off the Mikveh water from his body, as will be explained next.
Drying oneself after Mikveh: Based on the teachings of the Arizal one should not dry the Mikveh water off his body after immersing. Practically, we are particular to dry ourselves with a towel after immersion and one who desires to follow the directive of the Arizal is to leave some part of his body not dried. It suffices for one to leave his feet wet in order to fulfill this directive.
 See Piskeiy Teshuvos 88 footnote 16; Sheyikadesh Atzmo 63:5; Heichal Besht 15 p. 183
 Reishis Chochma Shaar Ahava 11:39
 Mishmeres Shalom 2:1; Siddur Lev Sameich in name of Belzer Rav; Baal Shem Tov Al Hatorah Parshas Yisro 15; Likkutei Sichos 17:481 that some say this in the name of the Besht
 Likkutei Sichos 17:481
 Divrei Yatziv 1:56; See Siddur Or Yashar not to go if unhealthy; See also Igros Kodesh Rayatz 2:142; Igros Kodesh 13:441 [must ask Rofei Mumcheh]; 23:295; Shemuos Usippurim end of Volume 1 in story with Rav Yaakov Landau and Rebbe Rashab that Reb Hillel Miparitch did not immerse in a Mikveh one time because it was cold and was bad for his health, and a Chassid does not do things that are bad for his health; See Heichal Besht Vol. 14 footnote 39 that the Rebbe Raytaz writes that the Rebbe Rashab did not immerse when there were health impediments
 Igros Kodesh 23:295 and 13:441
 See Admur 606:11-12; Igros Kodesh 20:252, printed in Likkutei Sichos 7:315
 Admur 606:11; Rama 606:4; Mahariy Viyal 191
 Admur 606:12; M”A 606:8; Darkei Moshe 606:3; Maharil Hilchos Erev Yom Kippur p. 315; Sefer Chassidim 394; Reishis Chochma Shaar Ahava 11:29
The reason: As this symbolizes that the person has done Teshuvah and is now reborn as a new person. [ibid]
 Likkutei Sichos 7:315 that there are opinions who say: 2, 4, 5 [Peri Haretz Parshas Lech Licha in name of the Malach; Sefer Habesht Parshas Yisro];13, 14, 39 [Shalah Shaar Haosiyos]. The Arizal would immerse 2 times on Erev Shabbos, once to remove the mundane garments of his soul, and the second to attain the holy garments of Shabbos. If one is a Baal Keri then he is to dip three times in total according to the Arizal. [Kaf Hachaim 260:6]
 The Rebbe concludes “Who am I to arbitrate between all these opinions”. [Rebbe ibid]
 As one of the purposes and reasons behind the daily immersion is as an act of Teshuvah; So rule regarding Yom Kippur: Beir Heiytiv 581:16; Mateh Efraim 606:8; and so is implied from Admur 606:12 that the custom on Erev Yom Kippur is to immerse also for Teshuvah purposes; See Likkutei Sichos 7:315
 Likkutei Sichos 7:315
 Rabbi Groner in Heichal Besht 17 and Maaseh Melech ibid footnote 5 and that the Rebbe told Rabbi Groner that by us the custom is to immerse nine times, straight, flat, straight, three times, and that so also directed the Rebbe to the Baal Tokeia of Musaf Rav Yosef Tennenbaum
 See Yimei Melech 2:631; Maaseh Melech p. 101; Rabbi Groner ibid
 See Sichos Kodesh 5738 1:396
 Maaseh Melech ibid footnote 5
 Eshkol [Tefila 1]; Mamar Mordechai  1-2, brought in Shaareiy Teshuvah 88:1; Biur Halacha 88 “Vechen”; Shulchan Melachim in Halacha Lemoshe Dinei Tevilas Ezra 12; Ashel Avraham Butchach Tinyana 88; Piskeiy Teshuvos 88:8; Sheyikadesh Atzmo 63:2; Leaning opinion of Minchas Chinuch 180:4
Other Opinions: Some Poskim rule that a Chatzitza invalidates the immersion of a Baal Keri. [Amudei Eish Kuntrus Beis Tefila 12; opinion of Gadol brought in Minchas Chinuch 180:4; See Reishis Chochma Shaar Ahava 11:29] The Shaareiy Teshuvah ibid leaves this matter in question. The Ashel Avraham Tinyana [Butchach] 88 implies that initially one should remove a Chatzitza [The statement brought in Piskeiy Teshuvos ibid footnote 57 that the Ashel Avraham holds it does not even contain a Midas Chassidus is inaccurate] The Rashba [Yevamos 47] rules that a Chatzitza invalidates the immersion of a Baal Keri. Nevertheless, this only refers to one who needs to immerse Biblically, such as in the times of the Temple. However, for the immersion of Takanas Ezra even the Rashba agrees it is not necessary. [Shulchan Melachim ibid]
 See previous footnote “Other Poskim”
 Piskeiy Teshuvos 88:8
 The above statement of the Poskim that validate the immersion of a man even if he has a Chatzitza refer specifically to one who is immersing do to being a Baal Keri. However, when immersing for Teshuvah purposes, as is done on Erev Yom Kippur, the Poskim ibid write that one is to remove all Chatzitzas. This arouses a question regarding those that are accustomed to immersing daily, irrelevant of being a Baal Keri, due to extra purity. Perhaps they too should be careful with all the laws of Chatzitza. Practically, the custom of Chassidim is to shower prior to Mikveh, although they are not particular to cut the nails and brush the knots out of the hair during this time. Vetzaruch Iyun.
 Shut Migdolos Merchakim Y.D. 57 that it is more stringent than Tevilas Ezra; Custom of Reb Itcher Der Masmid
 Ben Ish Chaiy Netzavim 3; Kaf Hachaim 581:84; This is also required based on the teachings of the Arizal. [Kaf Hachaim 260:4]; M”E 606:8 regarding Erev Yom Kippur
 M”E 606:8 regarding Erev Yom Kippur
 Igros Kodesh 11:400
 Igros Kodehs ibid; See Chagiga 18b; Chulin 31a; Michaber Y.D. end of 198
Other opinions: Some Chassidic masters write that immersion without Kavana is worthless and even detrimental, and hence discouraged the daily immersion. [See Heichal Besht Vol. 14]
 Siddur Im Dach Kavanas Hamikveh
 See Reishis Chochma Shaar Ahava 11:2435; Siddur Im Dach ibid Kavanas HaBesht; Heichal Besht ibid
 In Shabbos 14a the Gemara states that one may not shower his head and majority of his body in water after immersing in the Mikveh. It is disputed as to what form of immersion this decree applied to, as brought in Shulchan Aruch 202:75
 Shevet Halevi 7:33; Piskeiy Teshuvos 88:8; Even regarding a Niddah the Rama Yoreh Deah 202:75 rules that only some opinions are stringent and so is the custom. However, others argue on Rama and rule doing so is permitted even by a Niddah. [Gr”a ibid; Darkei Teshuvah 202:332; Lechem Vesimla 122; Birkeiy Yosef 202:75 in name of Rabbeinu Yeshayah 152 and Peri Hadama]
Other opinions: Some Poskim rule one may not shower after Mikveh, just as is the custom regarding a Mikveh. [Tiferes Adam 1:28]
 Shevet Halevi ibid concludes that it is proper to be stringent; Piskeiy Teshuvos 260:1 in name of Shevet Halevi that so is the custom of the world to avoid doing so despite it being allowed from the letter of the law. See also Tiferes Adam 1:28which rules one should never shower after Mikveh.
 Based on Kaf Hachaim 260:8
 As the water is considered holy with the spirit of Shabbos and it is thus proper for the body to absorb it. [ibid] However see Kaf Hachaim 260:8 which writes that this only applies if one did not immerse more than once, otherwise he has already washed away the main Mikveh water. Nevertheless, he concludes that even these second waters of the Mikveh has some holiness and is thus not to be dried.
 The holiness of that leftover water from the Mikveh will then subsequently spread to the rest of the body. [Rebbe Shaareiy Halacha Uminhag 1:131]
 Shaareiy Halacha Uminhag 1:131