1. We read books together (Ina was 3, whereas your oldest is nearly six, but I bet they'll still be helpful). We read:
Sophie and the New Baby (shows bfing, talks about how much work the baby is, and Sophie at one point yells, "I don't want that new baby!" But then she is the one who teaches the baby to smile, etc. Mothering recommended this awhile ago, it's out of print now but you should be able to find it at the library. I think this helped Ina a lot with the adjustment, because she knew that it would be hard sometimes, but that it would be worth it.
The Berenstein Bears new baby book....
Will There Be a Lap For Me? which I liked because it also showed bfing, and was much more urban and multicultural; the little boy is probably closer to your dd's age (whereas Sophie is probably closer to 3). Also talked a bit about how baby seemed to get so much attention, but ends with big brother sitting on mom's lap watching the birds at their new bird feeder.
We talked about how things would be after baby arrived, that baby would need to nurse alot, and sleep alot, and not be very playful 'til several months later.... it helped that my sisters each had babies about six months in advance, so Ina was able to see how newborn babies were.
Once SJ arrived, we realized that Ina needed 1:1 time with just me -- so I think SJ was probably only a couple weeks old when I started taking a half-hour walk with just Ina every afternoon. It made a world of difference for her!! She had been acting out a bit before then (not towards SJ, just crabby and clingy and destructive-ish). It's important that there still be some solid attention from mom; dh was giving Ina tons of attention, as were the grandparents, but she needed the reassurace of time with me. We did this at bedtime too -Ina got separate time from me, while dh cared for SJ and I read some stories to Ina.
If your little one ends up being another girl, I highly recommend a book called Big Sister, Little Sister (by an author named Pham). It is a really sweet picture book with cute illustrations, about the differences (and similarities) between two sisters.
Good luck! I do think that siblings being at the birth, depending on the sibling, could be also helpful. Ina really wanted to be there but we didn't think she was old enough.