I keep some card and letters. Especially cards that have little messages written in them, and letters that express or reveal something about the person who wrote/sent the card. If it doesn't hold any meaning for you then you won't miss it. Some times I pull out some old cards & letters I have saved. I enjoy re-reading something my great aunt wrote or my mother-in-law both have been gone for more than 26 years.
But it is easier to let go of things I have more than what I need, or that I never use.