My pennies' worth:
Support: make sure DH/OH is fully supportive of your choice and does the things apart from feeding (nappy change, wind, bath etc).
Look after yourself: Someone told me to have a little box next to feeding chair/in bed with large bottle of water, magazine, breakfast bars, ipod etc. DH/OH to support with housework. BF'ing takes it out of you so make sure you eat as well as you can (I lived on sandwiches for what felt like weeks but with a bit of salad, fairly healthy) and drink plenty of fluids.
Clothes: other posters have mentioned the mess, full agree. Invest in some good nursing vests/tops for day time and some cheap vests for night time (primark do ace hidden support ones that fit breast pads in)
Research: like some others have put, read up on how breastfeeding works (on demand etc) Kellymom website is fab.
Do what's right for you: if you need to co-sleep, do so; if you need to cluster feed, do so; if you get comments which upset you from family/friends/'creating a rod for your own back brigade' - Ignore them. you will get this, I did from some family member (most were supportive) but I just switched off to it and carried on doing what felt right to me.
Agree with other comments on here. Great thread for first time mums new to breastfeeding.
I got through tongue tie of dd, mastitis and sore nips in the first two weeks and dd is 5 months now and we are still going strong.
All the best