My dd1 was posterior, and also big (9lb12) and it did make the labour more prolonged and difficult.
The advice is usually to get down on your hands and knees, scrub the kitchen floor, lie over a birthing ball, anything to stay in that position in order to try to turn the baby. The other thing I was told (I was keen to avoid a second and third posterior baby so did things to avoid it on the other two occasions) is not to let your knees go higher than your hips - eg sitting on a low sofa or in the car. In the car, sit on a pillow. In the house, sit backwards on a hard chair like a kitchen chair, or on a ball if you have one (I loved mine). Baby could easily turn loads of times again before birth. My dd1 was anterior when I went into labour and went for a spin!
I don't want to worry you with awful stories about it as I then had 2 'normal' labours which were much much easier, but I'm not sure if it was because she was OP or because she was my first and largest.
I ended up with a ventouse as she wouldn't come and she was born face up (technically known as 'face to pubes' yuk! But someone here told me she's heard it referred to as 'stargazer' adn that sounds much nicer). I chose not to have an epidural as I knew it would make a cs more likely and I wanted to avoid that. But complications of all kinds are more common with a posterior position, including cs, instumental delivery, needing augmentation of labour. The weirdest thing was that I had no urge to push. I thought I did, I was pushing as hard as I could, but then when I had ds I found out what a real urge to push was! Dd1's head wasn't making proper contact with the cervix so I was slow to dilate and didn't get the pushy feeling. I had a tear - in the end they didnt have time for an episiotomy and I wanted to avoid one if I could anyway, which healed up well, but again I think her size had more to do with that than her position, she was on the 98th centile so not very likely yours will be that big!
All the best! In the end, whatever position baby is in and however you deliver, you will have a wonderful baby, and that's the important thing! I'll be thinking of you and watching the birth announcements thread!