Ages and stages
Newborns: Swaddle them in a solid-colored receiving blanket or leave them naked (if they start to wiggle, though, get ready for a shower). It's probably best to let them sleep. Place the baby on their back, and then position yourself directly over the baby. Make sure you're not looking up their nose.
3 months: If they can hold their head up, put them on their belly, with their fists in front of their chin for support. Get down on their level, on your stomach, and take the photos head-on. You want them to be awake to see their expression.
6 months: Once they're able to sit on their own, you can have fun with clothes and accessories. Act crazy and try to get them to laugh or smile. It might be a good idea to have someone else there to help keep their attention.