The most valuable advice I wish I'd had was to read anything you need to before giving birth - ie. breast pump instructions etc. It's really hard to read a book or even instruction pamphlet whilst grappling with a baby who can't support their own head etc.
A kindle (even if it's just an app on your phone). It's the only way you'll be able to read for a while and you will need something to keep you awake/occupied during long night feeds. iPad, even better for watching downloaded movies, iplayer etc, pleading for advice on MN.
"My breastfriend" breastfeeding pillow is an absolute lifesaver if you ever want the use of your arms again! You can also get an inflatable version for out and about (a bit harder to find, but possible on ebay). It straps all the way around your waist and sits like a table at your waist with a head rest under each breast for the baby. I managed to get up and get stuff out of the fridge as well as doing other stuff in the kitchen, answer the phone etc when I started using this, which I couldn't do at all before! Saved my back too. The widgey type pillows drove me nuts. They are so firm that every time you move backwards, the pillow shifts away from you and the baby falls down the valley between you and the cushion.
Find out where your local breastfeeding drop in clinics are. They can be invaluable, not just for supporting you through the difficult times of getting the hang of feeding but also to meet other new mums.
Determination, a strong will, a box of tissues (there can be a lot of tears in the early weeks, this is only natural and will pass - another reason it's great to try and meet others in your position at breast feeding drop ins). When people give you the "it will get easier" mantra believe them, it's true. Especially once they can support their necks and get the hang of feeding.
Cereal bars, water bottle, thermos coffee mug to prevent spilling well needed coffee on the baby.
Lots of back support. A donut cushion for the first few weeks if you're having a natural birth. All that sitting for hours on end, shifting position to feed correctly can be very uncomfortable on the stitches.