I have lost 50 lbs over the past year, So I feel that I can offer some practical advice from personal experience.
I recommend that you do not diet on Shabbos. I say this from a Torah perspective and also from a dieting one. Shabbos is mean to be a break, a day of pleasure and holiness. It should be different from the rest of the week. It’s ok to have a piece of challah or two at meals, a reasonable piece of kugel at Kiddush, or cake for dessert as long as you are disciplined during the week. Also, it will help to elevate your eating, as a way of honoring Shabbos. As a matter of fact, once you elevate you eating to the point where you are eating for energy to serve Hashem, rather than pleasure, you will be able to maintain discipline in eating without having to pay “fines” to each other for breaking the diet. Also, I do recommend diet soda. It has helped me. It’s not a good thing everyday, but it can help you not to overload on too many empty calories during Shabbos meals.