When I started snailmailing again a few years back, I quickly realized I needed a way to keep track of my mailings. So I did the most basic thing - I grabbed a little notebook and jotted down the letters, postcards and packages I sent and received. I also wrote down how much money I spent on postage, as I was interested to know how much I'd be spending (now I believe it's better not to know... I want to enjoy this hobby without realizing what I'm spending :) )
Later on I started stamping the dates of receiving and sending on the envelopes.
I didn't really like that however and didn't do it very often - while I then did know whether or not I replied to the letter I had in my hands, I like the letters the way I receive them and rather want to keep them the way my pals intended them to be, and not leave any marks on them myself.
So tell me - how do you keep track of your incoming / outgoing mail?