Congratulations! Everything everyone else mentioned is right on. The Adventures in Tandem Nursing has been my support, in addition to my La Leche League group. I am four months pregnant. When we found out, my son was still nursing numerous times a day. He has weaned to nursing only 1-3 times per day most days. He can't get the flu shot due to egg allergies, so I, too, am hoping to get us through flu season.
Sore nipples have been a hurdle for me, but knowing what my reasons are for continuing to provide this to my son keep me going, much like overcoming obstacles in the early breastfeeding relationship.
There are many benefits to continuing to nurse the older one, if you wish to, with the second one. One is that the older can help with engorgement issues early on. The other, more important one to my mind, is the reports of there being a closer sibling relationship and bonding, as the older child still gets bonding time with mom while nursing and doesn't feel as "displaced".
There was also an excellent article in Mothering magazine maybe three issues back titled "Unweaning Georgia" about a mom whose 3 yo had weaned before the baby, but there were so many issues after the baby mom decided to try nursing the older sibling again, and the relationships all improved immediately.
La Leche League (www.llli.org) has support forums and meetings. Mothering has support forums also (www.mothering.com).
I have found it more important to set limits for myself and respect my body. I have been more tired and nauseaus this time around, so have been taking frequent rest breaks, or even resuming the early habit of sleep when the baby sleeps, meaning taking naps if/when he does and going to bed some nights at 7:30 right after him. Exercise has also helped, which I wasn't doing consistently before getting pregnant. Eating food, let alone healthy food, was an issue first trimester, but now that the nausea is going away, I'm finding it easier to resume previously better eating habits.
Good luck in your pregancy and nursing relationship.