Assuming you are asking about WDW with MS and will limit my suggestions to that:
Although I need a
scooter as walking is difficult for me now, you may want to consider one at least part of the time. It is a great way to conserve your energy. It also allows you to get back to your hotel (or even a shady spot) more quickly when you need a rest.
Do not understimate how much the heat and sun will aggravate MS. It saps your strength, can cause new saymptoms, and aggravate pre-existing symptoms. That being said, I do WDW regularly and LOVE it, but I have learned what concessions my MS-body needs.
LISTEN TO YOUR BODY. When you are tired STOP AND REST. Pushing yourself is not a good idea. I nap everyday at WDW.
Hydrate, consider colling devices like the neck wraps and bandanas.
Ask forhelp when you need it. MS is often an invisible disability, yes, some people will judge you not knowing you have an illness, but ignore peolpe.
Hope you have a fabulous trip.