A pillow for your back!!!! I traveled 14 hours from NC to MI at 30 weeks and I was miserable on the way up because I didn't bring a pillow. We had to stop at 9 hours and rest for the night because not only was my back killing me, but my hips got really tense and I could barely stand up straight!
Pack food and beverages. I bring a pillow and comfy blanket. I also make sure to dress in cozy clothing. Oh and this might sound silly but NO BRA! LOL Way more comfy without one. All of this of course if you are the passenger and not the driver. If you are the one driving, disregard the pillow and blanket. Safe travels!
Make sure that you get out and walk around periodically too. It's important to keep the blood flowing when you're pregnant. It helps prevent clots. =)
Maybe a pillow for your back and definitely many pitstops to pee. That's all I can recommend. Oh and plenty of snacks.