Mine actually litter-trained herself, she still poops whereever she wants, which isnt a big deal. But what happened with her is this
She often plays with her waterbottle, (which is 20 oz might I add) more like empties the whole thing than asks for more, so we put her watterbottle over the litterbox in her cage so we could clean it and give her fresh water easily. Then her next idea was to move the litter box so we put an electrical tie through a small hole in the top of the pan and connect it to her cage. Anyway, somehow on her own, being the neat tidy pig she is realized if she pees in the litterbox it will get changed with the "pool" of water she makes. This is one smart pig, she dosnt like her cage wet AT ALL and realized she could just pee in there after seeing it come back clean almost every day. I dont think she pees anywhere else anymore. Then the water thing was becomming a real problem (and we only did this once) we filled it up all the way (which is alot more than usual because we usually fill often just to halfway) and she played with half of it, and the next day she played a bit with the last half but much less because nobody came to give her more to play with so she realized she couldnt (dont worry we gave her a full thing of water that night). It was never a problem again, this lady practically littertrained herself, all we did was change the litter.