Change-up your workout every 6-8 weeks.
Our bodies have this amazing capability to adapt to our surroundings and rigors that we put ourselves through. While this is great from an evolutionary standpoint, it is problematic when our workouts no longer become effective because we have been performing them too long. Buford, Rossi, Smith, and Warren (2007) found that changing up your workout every 6-8 weeks will halt any performance plateaus that a person experiences, and will allow further positive physiological change to take place. This can be as simple as performing 3 sets of 10 instead of 5 sets of 5 during resistance training, or working on hill training instead of running on the flat track.
Physical Wellness Educator from the Style Network's Too Fat For Fifteen: Fighting Back, John Taylor.