Several years ago we had a juggling "craze" at my otherwise very stodgy corporate office. We all juggled constantly on every little break and several of us got pretty good at it. By the end a few of us were doing harder 3-ball tricks like mills mess and passing balls back and forth to each other. It was super fun. Honestly while specific tips can help, I think it mainly comes down to persistence and getting ideas from others, youtube, etc. Some ideas will click, some won't. Doing it with friends everyday helps too. It just takes time to get a really consistent throw dialed in, especially under pressure, but once you do 3-ball gets quite easy and then you can focus more on goofing around and trying new things.