A meta-analysis of 94 independent tests: people who decided when, where and how they would act reached their goals far more often than people who only set the goal.
Gollwitzer, P. M., & Sheeran, P. (2006). Implementation intentions and goal achievement. Advances in Experimental Social Psychology, 38, 69–119.
Implementation intentions
“I want to get fit” rarely survives a busy week. “Gym, Monday, 06:00” does, because the decision is already made when the moment comes. An effect of d = 0.65 is considered medium to large in psychology.