When my two oldest sons were ages 1 and 4 I decided to write down things they did. I would capture brief snippets of the items on a Palm Pilot (at the time - later a Blackberry and finally an iPhone). Each month I would look through the brief notes and write a few sentences about each item. It could have been a quote, an event in their life, or just one of my observations.
I never missed a month. And in any month I probably never spent more than an hour in total on the effort.
Today I have three sons, and hundreds of pages of wonderful memories.
What can you do with a little effort consistently applied?