“Every day, expect that day to be your last. And act accordingly. Do teshuva. So say the Seforim HaKedoshim.”
The alter of Slabodka said that before 30, one should not contemplate death, etc, bec it subdues the natural youthful exuberanc that younger people have. AS far as teshuva for preparation for your death, there is an axiom in halacha that someone who is aliva and well has a chazaka that he will live for at least one year more – so if he goes missing and we don’t know where (excluding, for example, if he was captured by terrorists who have the annoying habit of beheading captives on live video) or why or other details. Therefore, it is brought down in seforim that mitzvos which we do only once a year, one should be extra machmir for bec there is no chazaka that he’ll be around next time, and there is a big inyan mystically which we hold of that the last time of any mitzvah is in many ways the most significant and definitely has the most potential power. However, to do teshuva every day as if expecting imminent death for us bazman hazeh is probably wrong bec after the first few times it becomes rote and insincere, and if chas veshalom the real time of imminent death rolls around, it will be much harder to say it sincerely. And stam, insincere vidui is itelf a sin, and we even say an al chait for it on yom kippur.