Have a look at the forum diet guides here
Diet Guides
The two things that are generally advised are restricting the amount of nuggets and filtering water to reduce calcium - or in my case we use low calcium bottled water. This can really help. My vet's mantra is "keep everything flushing through..." so I increased the number of water bottles and repositioned them: the nuggets are served near the drink bottles as they drink when they eat them (mainly in the evenings) but I have bottles close to the hides where my pigs sleep so they don't have to wander out in the night to drink - they just tip their head out.
I do give Romaine lettuce and they like it. Iceberg is to be avoided - it apparently lacks nutrients - there might also be another reason (I've forgotten right now!) Mine are very fond of cucumber. My approach is to rotate the veggies so they tend to get lettuce, cucumber and bell pepper each day but other stuff as and when. We also give a lot of grass - it keeps the diet nice and wet.
Now, my old boy threw a giant stone this summer at the age of 5 1/2, and now has another (age 6) but I think these are to do with ageing as he's been OK up till this point. I've had girls young and old in the past with smaller stones and some have passed them and some have had them 'extracted' (ouch! but over quickly) but one thing I will say is that if you are seeing symptoms such as wet bottom end, hunching and chirping while peeing, blood-tinted pee etc check for UTI. We've had UTI more often than stones. If you are just seeing 'milky' or gritty pee with no other symptoms that's pretty normal for pigs - if it's all the time then yes, you might be at increased risk of stones or sludge and can look at their diet. But pigs absorb all the calcium they eat and pee out the excess - so some white patches are typical. So so you have any symptoms other than milky pee? And how old is your little lady?
(Off to bed now so will check thread tomorrow

x)