You really can't. Appendicitis is inflammation of the appx secondary to an obstruction. So the appx is just a little vestigial piece of bowel that communicates with the large bowel. Therefore it has an open lumen (tunnel/tube) to the bowel.

Anything that will cause a blockage along that lumen to the large bowel will cause the appendiceal secretions etc to build and clog up, creating a perfect environment for bacterial proliferation - hence the 'itis' or inflammation.

You can't prevent those blockages, which most commonly occur due to the development of 'appendicoliths', which is essentially just a calcified buildup of faecal matter/mucous secretions etc. Although I've seen a case this year of an appendicitis secondary to a mass which caused the obstruction, on a fukking 11 year old. 11 year old with a neuroendocrine tumour, extremely rare, extremely lucky we got it early.