After living on this diet for over a year, and finally getting my lab results within normal boundaries of potassium and phosphorus levels, I've decided nine-tenths of success is in portion size and moderation.
If you don't overdo it on any one thing, you can allow yourself a little of the "avoid" foods like tomatoes and cheese. Use them sparingly. But if you allow yourself to eat these foods once in a while, you won't feel so deprived and you'll be able to stick with the diet a little longer.
Invest in a small digital kitchen scale, available at just about any kitchen department for $20 or less. I use it to weigh out my meat portions, even on a sandwich. I can control what I'm eating and I can keep track of my intake better. Eating out is a whole different challenge --- especially when the typical portion of meat is 3 to 5 times more than what I'm supposed to eat. I try to eyeball the portion size. And I take a lot of food home with me.
In general, use common sense. And don't beat yourself up. If you overdo it one day on high-potassium food, try to make up for it the next day by making low-potassium choices. The same with the high-phosphorus foods.