Choose a time of the year (birthday, Christmas, New Year's, or another day that has some significance to you) and take a picture of your tot in the same spot year after year. Maybe you have a favorite chair or an antique piece of furniture that you know will be around for a lifetime that can serve as your marker.
I put my daughter in front of her rocking giraffe before she could even sit up and took her photo. A year later, I took it of her standing next to her long necked friend. For as long as I can remember to do it, I plan on taking that picture and enjoy watching her grow like a weed.
Do you already do something like this?