Water and protein. Not JUST water and protein, tho...lol. It takes some time to find what works for you and your body chemistry. Just ask yourself " Does this make my body feel strong and is it sustainable?". I need carbs for my brain to function so that's how I roll, but I find that increasing my protein and water really cuts down on the bad carb cravings without even thinking about it...particularly in the mornings.
I stopped sodas of any type, no crystal lite type of stuff in my water, no sweets except when my grandsons visit, increase veggies, fruit, and whole grain, cut back on how much milk drink, and increase water. I also try to follow the Mediterranean Diet. A good book is The Idiots Guide to the Mediterranean Diet.
I can't give you my "number one" tip because for me it was a combination. This is what I did to lose 70 lbs: 1) Completely eliminate refined starches and sugar (including alcohol) 2) Restrict fat to no more than 25% of my daily calories 3) Eat no more than 1,500 calories/day, spread out over 3 meals and 1-2 snacks 4) Eat lots of vegetables, fiber, and lean protein 5) Exercise vigorously AT LEAST 30 minutes/day 6) Eat mindfully: always at the table, not in front of the TV or while reading 7) Tell people close to me what I am doing; it keeps me accountable 8) Track my meals. 9) Set small goals for specific benchmarks, e.g. "Be under 200 lbs. by Memorial Day" or "Be down to 180 lbs. by ____'s birthday" etc. 10) Reward yourself with a pretty new outfit or shoes when you've achieved the really important milestones.
Everybody is different, though, so what worked for me might not be right for you. Still, think about what you CAN do and what you're WILLING to do, and draw up a list or plan for yourself; that may make it easier to persist. Good luck!
The journey of a thousand miles begins with a single step.