Here are some things you should know about your milk supply:
Supply and demand: In the beginning, your body may make more milk than your baby needs. But very quickly, it adjusts to make how much your baby drinks. In other words, it works on a supply-and-demand basis, supplying your baby with as much as he demands. It will even adjust as your baby goes through growth spurts or short periods when baby is not drinking as much.
Filling baby's belly: When baby is drinking, you'll see his jaw working all the way back to his ear, and hear and see him swallowing. A nursing session should last at least ten minutes, and afterward your baby should look sleepy and satisfied -- or even drunk on milk!
The importance of the pediatrician: Keep baby's well-baby visits -- among other things, the pediatrician will weigh your child to make sure he's gaining weight, a sure sign that breastfeeding is providing the nutrients he needs.
The role your diet plays: Making milk can take a lot out of you! Specifically, it requires water, so you'll need to drink even more than you did during pregnancy. Keep a water pitcher on hand and drink all day long, but especially during nursing sessions. Also, eating healthy foods will give your body the tools it needs to produce a healthy milk supply. One big bonus is that you should lose your pregnancy weight without too much effort!
Boosting production: Sometimes women truly aren't making enough milk and need to boost production. The standard advice is to simply nurse more often, or even add some pumping sessions into your day. This signals your body that there's a greater demand, and so it will up the supply. Beyond that, there are some herbal remedies that work for some women, such as teas and supplements made with the herb fenugreek. (Fenugreek should not be taken during pregnancy.)