Easily prevent evolutions
Most Pokemon evolve once they reach a certain level requirement. However, to prevent a Pokemon from evolving, during the Evolution animation, hold B to stop the evolution. The Pokemon will evolve at the next possible level-up, if all requirements/conditions are met. Normally, Everstones (which are rare) were the only way to stop an evolution in previous Pokemon games. They are still available in Pokemon Sun/Moon. For example, you can get one by defeating Ilima in his house in Hau’oli City. Ilima, the Trial Captain, goes home after you defeat the Melemele Island Kahuna.