The guide to how much fluid to drink while nursing is to "drink to thirst". There is no evidence to show that forcing fluids will increase your milk supply. (Another "old wive's tale".) Drinking too much water (over 12 glasses a day) can actually decrease your milk supply. The current recommendation is to drink 6-8 ounces of water each day. The problem is that when you are a new mom, you often forget to take care of yourself while you are so immersed in taking care of the baby. Try drinking a glass of water or juice whenever you sit down to nurse your baby. Most mothers find that they are thirstier when they are nursing. If you produce large amounts of pale urine and are not constipated, then you are probably drinking enough. If your stools are hard and dry, and your urine is concentrated and dark, then you may need to increase your fluid intake. Remember that it is normal for your first void of the morning to be darker and more concentrated than at any other time during the day.
A good rule of thumb is to take in 200-500 calories in excess of what you needed to maintain your weight before you were pregnant while you are nursing. Breastfeeding doesn't make you gain weight - in fact, you use up calories when you nurse, and helps you get rid of extra weight more quickly while you are nursing. Your body stores up fat during your pregnancy to provide the extra calories needed for milk production. It is easier to lose lower body fat (hips, buns, and thighs) when you are breastfeeding compared to the mom who is formula feeding. (YAY!)
If you are a vegetarian, you probably will not need to change your diet unless your diet doesn't include any animal protein (vegan and macrobiotic diets). If you do not want to consume any animal products, you should consider taking a vitamin supplement containing vitamin B12. If you don't eat any dairy products, you should make sure to get enough calcium. I recommend that every nursing mom continue taking prenatal vitamins for as long as she lactates.
Just like "drinking to thirst", you should "eat to hunger" while nursing. Studies have shown that most nursing mothers tend to lose up to one and a half pounds a month for the first 4-6 months after giving birth, and continue to lose weight at a slower rate beyond that time. They tend to lose more weight than formula feeding moms who take in fewer calories. If you want to lose weight (and most of us do - how depressing to still be wearing those old maternity clothes we got so sick of after the baby is born…) plan to do so slowly. It's best not to try to aggressively lose weight during the first couple of months after birth, because during that time your body needs to recover from childbirth and establish a good milk supply. Losing weight too quickly (more than one to one and one half pounds per week) has been associated with the release of environmental contaminants stored in body fat into the milk.
Begin your weight loss program slowly when you are breastfeeding. Increase your activity level and eat less fat and sugar and more fruits and vegetables. Try to take in at least 1,800 calories each day, and definitely no less than 1,500 calories. Even moderate dieting during lactation can help you lose 4-5 pounds each month, but don't expect to lose body fat until about 2 weeks postpartum. The weight you lose immediately after birth is usually fluid loss. Mothers who breastfeed more frequently lose weight faster than mothers who nurse less often, and mothers who nurse for shorter periods of time tend to lose weight more slowly than mothers who nurse longer. You may get lucky and find that you can eat more than you ever could before and still lose weight while nursing. That was the case with me, but I did find that eating a whole container of Ben and Jerry's before bedtime every night kept me from losing weight as quickly as I would have liked.