I breastfed exclusively for the first 8 months...and if I had it to do over again I would have done so for the first 12! My daughter is now eleven and still one of the healthiest kids I know - the doctor hasn't even seen her for three years, and only this year because she needed a physical for school (NO SHOTS - another forum?)
Anyway, the importance here is keeping YOURSELF healthy. I would strongly suggest that besides your usual vitamins (unless you're making sure you get them all in your food), is to make sure you're getting enough NATURAL sodium.
The key to healthy milk is good hydration with natural minerals. I have overcome a tremendous amount of health problems (due to deficiency occuring during breastfeeding) by adding this Seasalt found at my local health food store to my water.
The general rule is 1/4 tsp of Seasalt per quart of water. We should be drinking half our weight in ounces of water every single day - possibly more during nursing or if you still feel thirsty, but definitely half our weight in ounces daily of the water.
IF you don't like the taste of the salt IN your water, you can take a "salt shot" (the salt in a shotglass of water) in one swallow and follow up with the fresh water. Be sure your water is pure (no fluoride or chlorine) - that will harm both you and your baby. Reverse/osmosis or distilled is the best water, and don't buy walmart's nursery water which has added fluorides - BAD for you and baby!!! The minerals are taken out, too, though, which is why you need to add them back.
BTW, table salt is bad. The salt I mentioned above is good. I literally reduced my blood pressure back to normal by doing this...and it didn't take long!
Trust me, I know this, like I said, my health deteriorated quite quickly once I realized how depleted I was. That was 11 years ago, and I'm still working my way back up the ladder. This doesn't need to happen to you!
Another good forum to follow for health is the Iodine forums.