Avoid refined foods.
Avoid too much salt and take it easy on the sweets but don't deny yourself a treat now and then (or every day if you want).
Forgive yourself for gaining the weight in the first place and forgive yourself for every thing and for every way in which you're not perfect.
Love yourself ... or at least keep a check on the "self-hater" ie; the one who judges and condemns yourself for being who you are. Tell it to leave you alone or to shut-up.
Basically, aside from the emotional and psychological reasons that people eat too much, it really does come down to eating healthy foods and getting enough exercise or enough daily activity.
20 minutes of aerobic workout a day is enough if you're not gung-ho about working out. Just get some sweat happening and get your heart rate up.
Finally, imagine your body as healthy and slim and imagine yourself as happy.