For a whole year I took a picture of her on the same day of the month in the same spot. I had set a reminder in my phone for it, so I wouldn't forget :)
The collage I made, for free, on www.picmonkey.com. An easy way to do it and there are a lot of different formats, backgrounds, colours and graphics to choose from.
The collage is a fantastic way to document baby's first year. Isn't it great (and a little sad) to see how quickly kids grow up!