Congratulations!!!
There are a number of things you can do in advance to prepare. Check out the breastfeeding books at your library for tips. I spent my whole first pregnancy reading just about the delivery which takes care of itself, and almost no time reading about how to care for a newborn -- which won't take care of itself if you know what I mean. You have plenty of time to change your focus, and you learned more than you realize with your first time around even though the breastfeeding didn't work out.
Next sign up for a breastfeeding class at the hospital where you will deliver. It will probably be free and only take a couple of hours at most, but you will learn A LOT (I know I did) and they are usually taught by one of the lactation consultants who will visit you when the baby is born. She can check your latch in person and give you positioning tips that she has seen work most often.
I would also encourage you to discuss the situation with your pediatrician. Some doctors are more supportive of BF than others. You have plenty of time to change physicians now if you need to.
Lastly, try not to worry. The odds of the next baby not opening wide enough to latch on are very small, and I am sure you two will do great. Your baby is very receptive to your mood. When you are stressed out, guess what? You won't be alone. And baby will breastfeed better when relaxed, you will have more milk when relaxed, it all makes sense when you think about it.
You can also search for a certified lactation consultant near you or contact the nearest chapter of the La Leche League for additional help and support. I know you will have a more positive experience this time just by the fact that you are doing your homework in advance. Good luck and take care!!
http://parenting.ivillage.com/newborn/nbreastfeed/0,,3x1p,00.html
http://www.kellymom.com/bf/start/basics/latch-resources.html
http://video.about.com/breastfeeding/Breast-Feeding-Latch-Position.htm
http://www.lllusa.org/
