a friend of mine recently told me that she had been putting off buying and writing in her baby book for her little girl… well now she is pregnant again and realized she needed to get going on her little girl’s book as well as her new little one’s! so, she had been writing all of her daughter’s milestones on post it notes and storing them in a shoebox, so that when she wanted to sit down and write in her baby book, she could be organized and know exactly what she wanted to write! you could also keep a little notepad on your fridge for each child and write down little tid bits about them, funny things they said and did and when you have the time, take those notes and record them in the baby book… keep a doc open on your computer, keep a notebook at your desk, but just write. i have found with my little 2 year old, i have already forgotten so much of what he has done and said already, simply because i didn’t write it down!!
so, just write it somewhere and you’ll be thankful later! 
