When my son was three weeks old, I took him to Brimfield with me (yes, postpartum insanity, but we survived) and there was a woman who pensively told me to cherish every day because it goes so fast. In the cycle of exhaustion that comes with a newborn, when I wasn't sure what day it was or whether I'd actually slept, time seemed magnified and endless.
She was right, of course, as I find myself with an almost 4 year-old who's lost all vestiges of babyhood, ready to head off to preschool for the first time. My husband has been feeling it as well and there's just the tiniest bit of sadness that accompanies this growth process (along with great relief that we'll finally have some uninterrupted time on our hands).
I've never gotten around to doing a baby book (though I have a box full of special items) and I'm not nearly as organized with the memories as I'd like to be. But one thing I did for the first year was take a photo of my son each month. I kept it as simple as I could, the photo taken in the same window (the only window that got light in my New York apartment) until he was 10 months old and we moved to New Orleans. I look at these now and it seems like just yesterday and such a long time ago. I'm so happy I did just that one thing.