You should shake up your routine about every 6 to 8 weeks. Even something as simple as changing the order in which particular exercises are done will help. The reason being that your muscles adapt to how you use them, they become more efficient. Over time the same exercise becomes less beneficial to you.
so if you're lifting, make changes. Change from moderate wt/medium reps to heavy wt/low reps for a few weeks. Change the timing. if you're lifting on a 2 count up, hold 1, 4 count down, change it to a 3 count up, hold 2, 3 down, for example. Anything to make it different helps.
If you're running steady pace distance, shake it up & mix in some sprints. If it's all flat, add some hills. Come up with a new routine, stick with it for 6 to 8 weeks, then change again. Even changing back to the previous routine works bc it's different now.