Eat Your Usual Diet! You can pretty much eat whatever you ate during pregnancy. What's really important is to get adequate amounts of dairy products and fluids so you can make enough milk. If you don't like dairy take extra calcium and if your vitamin d levels are low, take a good vitamin d supplement. As always take adequate amounts of fruits and vegetables and it is good to take a probiotic like acidophillus as well.
Common Sense. Dr. Tuakli's answer is very good. Also keep in mind that there may be foods that mom eats that bother the baby. One learns this thru trial and error. These should generally be avoided.