I toss away almost every letter in my first town, and I save the farewell letters from past villagers (regardless of how much I don't like them) in my second town.
I'll always keep the ones that special characters like the mystery mayor or my character's parents wrote. I also keep letters I've really liked and goodbye letters from villagers.
I keep letters from my dad & goodbye letters from villagers who have left. I pretty much toss everything else as I don't want my saved letter box to get clogged up. ^^;
I keep the letters that were from the post office (Only if they give me their exclusives items) and I try my best to keep the villager letters that moved away for nostalgia.
Everything else I just toss.
i save the ones from my parents, the guy who was supposed to be mayor, letters i find sweet/cute, and the one from isabelle, when she sent me the lantern.
I keep all letters from my parents, along with letters from villagers who have moved away. I keep some of the special ones from the NPC's too sometimes
i keep the letters my "parents" send me. i've kept the mystery mayor one. i'm planning on keeping moving away letters and also cute letters from my dreamies. c:
I keep a few from each villager, ones from special characters like Jingle or Snowboy, and ones that highlight my achievements, like earning a certain HHA score, or saving a certain amount of money. My mail storage is totally full now. >.<
I've only kept one from Francine, I sent her a lot of letters with presents in one day and she replied with "I was stunned by the volume of them at first, but I read all your letters. Don't worry, my dear. I'm not leaving you. And if one day I must, I'll still always be in your heart." I thought it was so funny and cute ^-^