When we went last year, I was just shy of 6 months. Here are my tips, in no order of importance (although I'm sure I'll forget some).
1. Build up your walking endurance and do it in a very comfortable pair of shoes. Then wear those shoes in the parks everyday, even when not "fashionable".
2. Bring a bath salt type thing for soaking your feet at the end of the day. That helped a lot.
3. Avoid any and all rides with a pregnancy restriction. It's really not that many and the risk, no matter how minor, just isn't worth it. One exception might be Kilamajori Safari.
4. Study the map for all bathroom locations. You won't ever be far from one, but it helps to know where you're heading in advance.
5. Be sure of any food cravings and food avoidances when you make your ADRs. Mainly the food avoidance.
6. Stay onsite. If you can stay on a monorail resort or WL. These are easy resorts to get back to, but really any onsite resort will do.
7. Make sure all your clothes are comfortable. I actually grew a size from the time we left till the second day.
8. Talk to your OB about it as soon as you can and again right before you leave.
9. If driving, stop frequently and stretch.
10. Eat frequently and stay hydrated. Duh.
Do a search on this board and you'll come across many more. Heck, probably more posted by me alone.