I’m sorry you’ve gone through it enough times to become an expert on the subject. 
My additional tips:
I bought several different flavors of Crystal Light (NOT RED), which I was told I could use. I flavored each “glass” of the prep fluid with a different flavor, so I could make a big deal of comparing flavors and deciding which I liked the best (or hated the least). That helped me to break the gallon down into more manageable doses.
The most important tip, though, was the one I got from the nurse ahead of time: CINNAMON GUM! In between doses, or even between sips, I’d nibble on a little piece of cinnamon gum to “cleanse my palate.” It was magical and helped to get the flavors of the prep liquid out of my mouth. Just don’t swallow it, because that wouldn’t look good on the imaging!
All the best to you and to everyone who is facing similar challenges. 
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