For most people, just getting the pictures, letters, an other memorabilia into a container in the garage is organized enough. For anyone interested in a way to further sort your memories for easier future access–writing a memoir anyone?–listen up.
Take out the various and sundry items you have kept for sentimental reasons.
Sort them into piles that make sense to you. I do this by era. I have a pile for each military base on which my family lived throughout my childhood, a pile for my single years, a pile for wedding and honeymoon… You get the idea.
Some people I work with do it by person. They keep all letters from their grandma in a pile, all pictures of one child in a pile, etc.
Anyway you want to split it up will work. Think about the categories you already have in your mind when it comes to these items. Chances are that when you think about old memories you think of them in groups. What are those groups?
Next, put each pile into a large ziplock bag and label that puppy. Put the bags in a storage bin and the next time you want to reminisce about a time or write a chapter in your biography about a person you can easily grab that bag and take a trip back in time.