I still save all the move-out letters, just to know, which villagers are lived in my town. I also save funny letters from my villagers or letters from my dad (because there are so rarely ;-; ).
I never keep the letters no matter who it is from. I find that there really is no use on keeping the letters. Come to think of it, I actually don't even read them lol.
In my original town, I saved the birthday party invites. Now I save nothing. In fact, I rarely read the mail I'm sent. I open it and then immediately toss it.
I used to save every single letter unless it was an exact repeat of another. Eventually it got to a point where I couldn't fit any more letters so now I only save the ones I really like, which is most of them. I currently have two empty boxes in compartment C.
I save one from every villager in my town just so when they move away I will remember that I had this villager once before even though I could probably see them again in mainstreet, but that could take awhile! And it's good for storing rare items that you don't need but want to keep at the same time
Just saved Muffy moveout letter with her picture in.
I was using the red letters for saved items but it was a huge pain figuring out where the item was.
I use a new letter and write what is inside. Just read and find the item.
Two players letters are full of items.
I time travel a lot and therefore get tons of letters. Rather than weeding through them and reading them all I just store them.. probably never to be seen again lol. Even the ones from Jack and stuff I just, THROW into the post office without evne bothering throwing them away.