we have had a little trouble with constipation, and found that sugar water is a great way to get baby to poo...
use a teaspoon of water to 4ounces of water and watch the miracle unfold. i use it a lot with our son and he goes pretty regular, but when he gets bound up, i like to give him relief.
also, i have heard that Gripe water can be helpful. you should be able to find it with the gas medicine in infant aisle.