As you might expect, in running a letter writing organization, I have acquired a large amount of pen pals. This is great! It also means that my address book can be a messy place, and can be tricky to keep up-to-date.
First, I keep a paper address book. I like the physicality of it. Sure, it takes a little teeny bit longer to find an address than it would if I kept a digital book, but that is not enough to get me to switch. I use pencil for the entries. It makes it easy to update. I had a previous book in my college years which swiftly ran out of room where I'd have to cross out inked addresses and write in a new one. (Mind you, I enjoyed seeing the record of places people had been, but it wasn't really helpful as a reference tool after a while.) I keep small Post-It notes of the address on top of the address entry. That way, if I finish and seal a letter, I can stick the Post-It note on it until I'm ready to properly address and mail art-ify the envelope.
Because I do get a lot of mail, entry into my address book is not immediate. I don't add anyone until the third letter I've received from them. No one (until now) knows that I do that, but I consider it something pretty special, to get added to book. The act of writing things in is very important to me.
I also erase entries. Don't be shocked. I'll tell you why. When I do my holiday cards, I make a point to write to everyone who has an entry in the book. After I've finished a card, I place a rubber stamp next to the entry with a FriXion stamp. As the months go by, when I get a letter back, I erase the stamp. (That's the magic of the FriXion stamps. They are erasable.) If, by June, I haven't gotten a letter in return, then the whole entry gets erased. You have to write a letter to get a letter. It works both ways. Plus, there's only so much room in my book.
This is the system I've developed that works for me. I'd love to hear about your system in our comments!