Acharei Mot(Leviticus 16-18)
Aaron shall come to the tent of meeting - he shall remove the linen vestments that he had worn when he entered the sanctuary... He shall immerse himself in the water in a sacred place and don his vestments. (Lev. 18:23,24)
On Yom Kippur while performing the service in the Kodesh Kodoshim (holy of holies), the Kohen Gadol would wear special white linen garments. Whenever he would don those white vestments or change into his regular vestments he immersed himself in a mikveh. One normally immerses when he's in the process of elevating himself. If so, why did the Kohen Gadol immerse himself when he changed back into his regular vestments - wasn't he decreasing his level of holiness by changing back into his regular clothes?
Every good deed is supposed to elevate a person and bring him to new heights. Every Yom Tov that passes is not another Yom Tov gone - it's another Yom Tov in your pocket. The special qualities of that particular Yom Tov are supposed to bring you to a different plane and make you into a changed person. All the commandments you perform should be on a higher level than before.
After the Kohen Gadol wore the holy white garments he became so elevated that even the regular service he performed afterwards was on a higher level. The regular service went up a notch and he therefore immersed himself before donning his regular vestments.
In life a person needs to keep climbing to greater heights. His understanding of Judaism needs to be constantly deepened and not remain stagnant. His performance in mitzvot and appreciation for prayer should be advancing constantly.