My piggies are resonably well litter trained - I change the fleece about once every 10 days, and when I spot clean each day there are only about 5-10 stray poops!
My theory is that it is to do with them previously being kept on hay/newspaper - I have litter trays back then (a cat one - no cat litter in though, and a small pet one that goes in the corner) but they peed and pooped everywhere! However, when I put them out for floor time they would only use the litter trays. So when I got their c&c cage and fleece I think they carried on with the same idea, using the litter trays!
My tips would be...
1. Have a rectangle litter tray where all their hay goes and with hay racks - big enough for two piggies to fit. If you want also use a normal litter pan in one of the other corners - sometimes one of my pigs is bossy and won't let the other in the big one!
2. Use hay and newspaper in the litter trays - piggies often like to wee on their hay! To make sure they aren't eating spoilt hay I clean the litter trays every day and use hay racks
3. Put litter trays in dark corners or corners where they already like to go!
4. Try making these corners even darker - Over their large tray I have a grid with fleece draped over.
5. To start with you can try putting some old hay/few poops in the trays to encourage them to go where they smell themselves!
6. If you do floortime, use the litter trays from their cage.
7. If you pick a piggie up for a cuddle - place it back in a litter tray, I often find mine want to have a wee straightaway!