There are several different ways to try to insure a healthy supply. The number one thing is to refrain from supplementing. The more formula is supplemented, the less milk your body will produce. Only supplement if there is proof that your baby is truly not getting enough to eat (i.e. lethargic, losing too much weight too quickly, low urine and/or stool output, etc.).
In many cases, the reason baby is not getting enough is because mom is not making enough. Assuming mom has not been supplementing formula, there are some ways to try to ensure that your supply stays up.
1. Stay hydrated! This is very very important. Have lots of water and milk (preferably skim-milk) on hand. Drinks full of sugars and sodium (like sodas or juices) will not keep you hydrated. Sodas typically have the opposite effect, making you feel more thirsty.
2. Eat. Don't pig out on anything and everything you see, but do eat healthy and regularly. Your body will burn about 500 extra calories a day from producing milk. So, skip the crash diets. If you are eating less than you normally would have before you were breastfeeding, you won't be eating enough to continue breastfeeding.Your body will not have the energy it needs to continue producing milk.
3. Oatmeal. Oatmeal has been known to help boost supply, so have that on hand as well.
4. Herbs and teas. There are several different herbs and teas that have been known to help boost supply. The common ones are Fenugreek and Mother's Milk Tea. These can typically be found at health food stores and vitamin stores.
5. Medications. I am not one to jump to medication, but there are some out there that increase prolactin which is the hormone that helps make breast milk. You would need to consult your doctor about this, and I would recommend exhausting all other options first.
6. Relax and rest (as much as possible). Stress can hinder your body's ability to produce milk. Cut out anything that is causing you stress, and try to get as much sleep as you possibly can with a new baby.
7. Feed often. People will try to tell you that you need to get baby on a schedule. This is counter-productive in breastfeeding. Feed on demand so that your baby can let your body know how much milk it needs to make. If you try to set baby to a schedule, then the baby may not be hungry or may be overly hungry. In these cases, your body will either make too much or too little. Watch for signs of hunger, and feed your baby whenever he/she wants to establish a healthy supply.
8. Aim to be healthy. Swap the fries for broccoli. Try to fit in some time to exercise. The healthier you are, the better your body will produce breast milk.
9. Don't smoke. This can prevent your body from producing milk. Not to mention it's not healthy for you or the baby.