I follow the No S Diet - no sweets, no snacks, no seconds, except on days that start with S (Saturdays, Sundays, Special Days).
Only one plate of food, three times a day. No sweets, no snacks, no seconds. Then on weekends and holidays (Saturday, Sunday, Special Days), I can have sweets, snacks, or seconds.
I've been following this for over a decade, and in my 40s, I weigh 5 pounds less than I did in college. I run about 40 miles a week, and am competitive in local hobby jogger races in the masters division.
Initially, I'd abuse the system by binging on weekends, but over time, that stopped. Now my S days aren't anything crazy, and I feel terrible if I overeat, so I don't.
The plan is sustainable (again, I've been doing it over a decade), because instead of saying I can never have (fill in the blank), I tell myself I can have it on the weekend.