The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Lech-Lecha, discusses a number of different Halachot relevant to Shabbat preparations (listen to audio recording for precise citation). He writes that one should take a haircut on Ereb Shabbat in honor of Shabbat, and although this could be done any time on Friday, it is preferable to have the haircut before midday. The Arizal (Rav Yishak Luria of Safed, 1534-1572) always made a point of cutting his hair before midday, whenever he cut his hair, and not only on Friday. Preferably, then, haircuts should be taken specifically before midday, when possible.
The Ben Ish Hai then mentions that one should cut his fingernails and toenails each Friday in honor of Shabbat. If one’s nails do not grow fast enough to be cut each week, he adds, then one should cut them every two weeks. After cutting one’s nails – both fingernails and toenails – one should collect the nails and flush them down the toilet. There is a form of Tum’a (impurity) that rests upon removed fingernails and toenails, and they could be harmful. One must therefore ensure not to leave the nails in a place where they could be stepped on, or on his clothing or shoes. If a nail fell on the floor and one cannot find it, he should thoroughly sweep the area, to ensure that the nail is moved. Nails are not harmful once they are moved on the ground, so by thoroughly sweeping the floor, which results in the nail being moved, one can avoid any danger that the nail might otherwise cause.
It must be emphasized that these Halachot apply to both fingernails and toenails.
It is a Misva to bathe or shower with hot water on Friday in honor of Shabbat. One should have in mind while bathing to remove the impurities from his hands, feet and face. The Ben Ish Hai adds that it is a special Misva to wash oneself with soap, which takes the place of the ointments which people many years ago would apply on their skin before Shabbat.
According to Kabbalistic teaching, it is proper to immerse oneself in a Mikveh every Friday in honor of Shabbat, as this prepares for a person to receive the additional sanctity bestowed upon us on Shabbat eve. During the first immersion, one should have in mind to purify himself from spiritual impurities. The second immersion should be done with the intention of rectifying the effects of anger. The third immersion serves to remove the "weekday garments" of the soul, as it were, and the fourth immersion removes other aspects of the weekday soul in preparation for accepting the special soul of Shabbat. With the fifth immersion, one accepts the special light and sanctity of Shabbat. Some people immerse three times, corresponding to the three components of the soul (Nefesh, Ru’ah, Neshama), while others immerse five times (corresponding to Nefesh, Ru’ah, Neshama, Haya and Yehida). In any event, while we don’t fully understand these Kavanot (intentions), what is important for us is that one should make an effort to immerse in a Mikveh on Friday as part of his spiritual preparations for Shabbat. This might not always be feasible, but one should, at least from time to time, try to immerse on Friday, and the effort which we make in preparing for Shabbat is itself significant, even if we do not understand the full Kabbalistic significance of these immersions.
The Ben Ish Hai adds that one should preferably immerse on Shabbat morning, as well, because one is endowed with another new soul on Shabbat morning, which is even more significant than the new soul received on Shabbat eve. It is especially important to immerse on Shabbat morning if one became Tameh (ritually impure) on Friday night. Some exceptionally pious men immerse twice on Shabbat morning – once to remove the impurity, and a second time to accept the new level of holiness. Others, the Ben Ish Hai records, immerse five times on Shabbat morning.
Summary: It is proper to cut one’s hair on Friday, preferably before midday. One should also cut one’s fingernails and toenails on Friday, ensuring to discard the nails after they are removed. There is a Misva to bathe with hot water on Friday, and one should make an effort to immerse in the Mikveh on Friday to spiritually prepare oneself for Shabbat. There is a custom to immerse again on Shabbat morning.