Focus on the positives. Make a list of experiences you enjoy the most when walking. It is best to do this after a particularly enjoyable walk. On days where you have little get-up-and-go read them over. Make a list of things that prevent you from walking then write down how you can overcome them.
Keep "walking" reminders in places where you can see them. Remember the schedule you placed on the fridge. Keep your running shoes next to the door.
If you walk in the evenings try walking as soon as you get home, before dinner. If evenings are a problem, try walking early in the morning or at lunch break. This can help energize your day.
Make and sign a contract with yourself, then set some goals. Try not to set goals for the too distant future. Make sure you set short term goals that you can achieve and measure. Set a goal for each upcoming week. Think of people who can assist you in reaching your goals.