Adapt and Be Flexible
There's a reason that major leaguers have a bevy of coaches, trainers and professionals mulling around them during every batting practice and pitching warm-up. Athletes are not finely tuned machines that can perform every day like robots. Just like anyone else, certain skills weaken and must be built up, while unexpected strengths can also be harnessed.
And while it isn't groundbreaking or even terribly interesting, the truth is pretty obvious: keep practicing. It's continual honing of your game that will make you a more capable player. Take a page from major leaguers: Even if they were able to maintain a consistent level of play and skill, the game changes around them. A pitcher goes at them with a knuckleball, or the old-time sluggers are replaced with quick hitters that slap a ball infield to get on base.
Beyond strengthening skills or changing routines, remember that your body changes and small modifications in stance or throwing need to be addressed and corrected as you age. Consult with a coach or someone with professional training to pinpoint any tweaks you can make to improve your game.