My wife, my mom, and I all have restroom-related issues, so I think we know every restroom in WDW,
Disneyland, and every mall on the eastern seaboard!
Here are a couple of my own tips:
1. If you see a bathroom, GO. You probably know this one already, but do not pass up an opportunity to go to the restroom when you see one. You never know when you are going to find an unexpected line in a restroom.
2. Have a map with you at all times, even if you where the restrooms are. Sometimes you may have to find an alternate restroom (lines, renovations, etc), and you may want a map to help you find the quickest route.
3. In an emergency, you can go to First Aid and ask to use the restroom there. I do this when I am having a bathroom emergency and there is a line.
4. This is my most important tip: Know how long attractions are and when you cannot leave them in the middle. Ellen's Energy Adventure is 35 minutes long! If you try to leave CoP, you will cause the attraction to stop and reset, making everyone else re-watch that scene. This will not make you loved.
5. Know your triggers - stress, certain foods, whatever. Take it easy. Check menus online ahead of time so you can find easy foods for unhappy tummy.