I'm probably just reiterating what everyone else has said but, here goes:
Try to go at a quieter time of year, if you can. This will reduce crowds and queues, and just generally make things a little easier on you. It is also often cheaper in the quieter times.
Use something like Tour Guide Mike, as Sue suggested, to avoid crowds as much as possible.
Use every restroom you pass, even if you don't really need to go at the time. All right so it may mean a lot of stops, but it could help you feel more comfortable and happy.
DON'T stop drinking. I know it may be tempting to limit your fluid intake, so you have fewer problems, but it could wind up making you very ill. Don't take any chances, keep yourself hydrated.
Use the Fastpass system for the longer queues. That way, though you still have to wait for your time to roll around, you won't be 'trapped' in a line.
Talk to Guest Relations and see if there is any assistance they can offer you. I don't know of anything, but they may be able to offer you a Guest Assistance Card with a useful stamp on it (see the FAQ for more details).
Consider taking a spare pair of shorts in a bag, just in case. Hopefully you won't need them, but it's good to have the option there, if you need it.
Good luck and let us know how you get on!
