K.C.
Pose them in the same chair every month and, if possible, put something large next to them to show how they change compared to it (like a huge stuffed animal). Also, if you can dress them in the same type of outfit, it helps show the difference too.
I didn't really know about this idea until my son was almost one, so we do every year on his birthday instead of every month of his first year. He sits in the glider in his room wearing a plain white t-shirt/onesie and plain blue jeans. It's amazing how much he has changed from his 1st to his 3rd birthday.