Suppose I'm able to perfectly preserve one (or all) of the Arbaah Minim from last year so that they are still in "mint condition."
Can I use them again this year?
Regarding a old Esrog - Shaalos UTeshuvos Maharil 5 says that it is impossible to maintain its wetness from year to year and therefore it may not be used. Rama Orach Chaim 648 based on this Maharil says it is definitely dry and unusable.
However the Bikurei Yaakov (Aruch Laner) says that he saw an Esrog that was over a year old which was maintained by being placed in an airtight container and left in a cold place which had wetness and was not dry. He feels that a Bracha may be said on such an Esrog.
My great grandfather (in S. Germany) used to save his Lulav from year to year in his coat closet. He would remove the outer leaves and the insides were apparently still fresh enough to be used.
For that matter, a lot of the Lulavim and Haddasim that are in the market now, have been in storage for many months.
The Shulchan Aruch (based in the Mishna) spells out what is considered "dry" for each of the 4 Minim. If it's not dry then it's Kosher.
The problem arises with frozen Minim; they rot a few days after thawing out.
Rav Wosner in Shevet Halevi 1:176 brings from the Bach that it would be mutar as long as it didn't change from its regular look,but one has to know that water from the freezer can pasul it because water makes it rot.see inside for the discussion of kavush.
Rav Moshe Feinstein in Igros Moshe OC 1:185 discusses whether an esrog that was frozen can be used next year sukkos. He writes that it is not considered kavush.He said one has to know if it is rotten or not inside he says that fruit usually go,bad when frozen,but the question is if it lasts a few days after taken out of frezzer was it rotten but not showing signs yet.
He brings the gemarah with Rava(Sukkah 36a) if it starts to rot inside or out,as a comparison, ,see it inside.It seems that one should not use such an esrog.
Rav Shmuel Kamenetsky in Kovetz Halchos holds that by a year old esrog even if wrapped up and preserved,one should still not use it(see footntes brings the discussion of the poskim who hold yes or no).Regarding a year old lulav if it is still green then it would be fine(however see footnote for a savorah).
Regarding an old lulav see Igros Moshe OC 1:184 ,I did not read it.