Negating their feelings by saying, "that's silly" or "you shouldn't feel that way" is counterproductive. Equally damaging is assuring them you understand just how they feel, when there's no way that you can (and an overweight person knows that, too).

Listen -- really listen -- before trying to offer practical advice. Find out the triggers that make that person want to indulge or the factors than prevent them from beginning an exercise plan. Know all of the new rules that will apply to the relationship, even if it means avoiding the old familiar places that evoke temptation or yearning. Only by discovering how you can be a part of your significant other's support network can you be a part of their self-esteem recovery.