I am going to start with a list things I found important to have to nurse my son.
1) A Nursing Pillow - Nursing can take a toll on your back. Having a nursing pillow can help you nurse in a much more comfortable way. I used the boppy until he was about 4 months old. I would recommend getting a boppy that comes with a slipcover. They are cheaper that way and then you can wash the slipcover.
Bonus: A boppy is also a great way to prop up baby for tummy time.
I also invested in some washable pads. You can buy some at Babies R Us but those aren't actually waterproof. I found some on Etsy.com that I really LOVED. Charis Baby Designs has great pads. I threw them in the wash, air dried them and used them OVER AND OVER AND OVER again. I think they are definitely worth the investment. I think they paid for themselves at least 4 times over. I only got one set the first time, I think I will get two sets next time.
5) "Sleep" Bras - I got mine from Target. I have seen that Motherhood and Medela makes them too. But these were great for sleeping in and laying around the house in. They were very easy to pull down when it was time to nurse. In fact, I found these much easier to nurse in than a nursing bra. And they were comfortable in the first few weeks that I was dealing with engorgement.
6) Nursing Cover - If you will find yourself in nursing in public and aren't comfortable with showing skin, you will want a cover. There are a lot of different versions by different brands. Some have pockets, some have tools to keep track of which side you nursed on. I just got a regular one from Target.