1. Always Be Learning
Learning is one of the best ways to protect your brain, hands-down. When you force your minds to stretch through taking on mental challenges, you prevent the breakdown of neurons and maintain stronger mental faculties. The greater the challenge, the better, which is why many older people opt to learn new skills: photography, coding, another language, etc.
You don't necessarily have to learn brand-new skills in order to take advantage of this tactic. For instance, simply learning a few new words through crossword puzzles or stretching your sudoku skills can do a lot to keep your memory sharp.