First, age matters when it comes to getting that perfect photo of your infant. Too old (older than 2 weeks) and you will begin to have trouble with posing as baby will be able to move about more freely. This doesn't mean you can't get great infant photos of your baby. It just means they may not be like those photos you see of the baby in the little wooden bowl, or with an arm out over the basket. It may end up more like the photo you see here in this blog. An amazing infant (sibling) photo, just not what you see in all the parent magazines.
Second, temperature is important! A cold baby is a crying baby, so keep the room warm. I usually have the parents turn the heat up to at least 78 to keep baby toasty and comfortable so that he or she will remain happy during the shoot.
Third, Infant sessions take a while! It is very hard to get the perfect infant photo package in under 3 hours. Baby will get hungry, need changed, get fussy and need to take breaks. This length of session will allow for all of those things and give you the opportunity to make baby happy again for the next few pics.
It's also great to be able to keep your baby in his or her own comfortable surroundings. I am a natural light, on-location photographer. I enjoy shooting in nature's studio. For infants, I can do them outdoors or in the comfort of their own nursery or somewhere else in their home (with good natural light source), which gives the infant that sense of familiarity, and gives the family control over those things I previously mentioned (temperature, feeding, break time). But studio or not, these tips will help you get the most out of your infant photography session.
Good luck and happy shooting!