I lost ~90 lbs (starting in August 2004) and have kept it off - with a 5-lb fluctuation cushion for times of vacation and/or stress. I work with almost exclusively overweight and diabetic patients.
Best advice for maintaining weight loss:
Decide what are YOUR special occasions and then have "special occasion foods" only at those times. Life is no fun if we try to swear off cookies or birthday cake forever, but we have to define what is truly a special occasion for ourselves. Example: When coworkers bring in some crappy grocery store cake, that's not special enough for me to eat. If it were, I'd be having somebody's birthday cake at least once a week! But when I special order a birthday cake from my favorite bakery for one of my family members, THAT'S worth it to me. I have a piece, enjoy it, and then don't have seconds. I send any leftovers home with someone else or take it to work where it magically disappears!
Keep in mind that a special occasion is ONE DAY. Christmas, for example, is a day when I eat whatever I want. But I don't start 2 weeks before the holiday and then keep eating for a week afterward.
Best advice for losing weight:
Life is exciting enough - we don't need food to make it more exciting. Automate your everyday meals.
Eat a little less at each meal than your usual. Enjoy your food and then stop eating when you're full or the plate is empty, whichever comes first.
Stop snacking! We don't work out in the fields or spend all day tromping around the woods in order find food in our society. We largely sit around on our behinds all day and snacking is not necessary for that.
Take a walk. You don't need spandex. You don't need a gym membership. Comfortable shoes and 10-15 minutes after each meal will give astonishing results in blood sugar, stress reduction, and also gives a handy signal that mealtime is over.