Nurse as long as you can! I found out I was pregnant again the day before my son's first birthday, and started weaning then (I'd planned to nurse two years). My morning sickness increased in direct proportion to how far along with the weaning I was. I later learned from a friend that nursing can stop morning sickness (I think she said it was because of the hormones involved, but don't quote me on that). If it happens again, I probably won't even think about weaning until the second or third trimester.
As a side note- expect your first to want to nurse again after s/he sees the baby doing it. Some kids will ask, nurse once if their mom lets them, and then not show any more interest at all. Others, like my son, will think that's permission to start nursing again. My friend is nursing both her toddler and newborn full-time, which is another option you have. She says it isn't hard, just exhausting (and she's thinking of weaning the older one).