The Psychology of Habit
Motivation gets you started, but habit keeps you going. Many people fail their fitness resolutions because they rely purely on willpower.
Start Small
Instead of completely overhauling your life, change one small thing at a time. Commit to drinking one more glass of water a day, or walking for 15 minutes. Once that becomes automatic, add another small habit.
Consistency Over Intensity
Working out lightly for 20 minutes every day is better than an intense 2-hour workout once a month. Build the routine first, then increase the intensity.