How many diets have you tried? Can you count them on more than one hand? If so, there might be more to the problem than just the unrealistic eating schedule they put you on that can never last. (That's why you do something like I talked here!) But here is another hint...your mind has to be ready to diet just like your body does. Even further, just like you need to hit the gym every day and exercise your body, you need to work your mind to be at it's peak to lose the weight. The key? Think like a thin person.
Everyone can admit to starting a diet and then having those thoughts such as "I can't diet when I'm stressed" or "I know I shouldn't eat this, but it's my birthday/Thanksgiving/Groundhog Day/fill-in-the-blank day". Then the diet quickly goes down the drain.
The first trick is to get the idea out of your head that once you are off the diet, you can eat whatever you want. Talk to anyone that has gone through a diet, and for it to be affective it must be a life style change. You might be able to cheat a bit more as long as you take more time in the gym, but never will you be able to go back to your ways prior to losing the weight. Thin people are aware of what they eat at all times, not in an obsessive way, but a way that keeps them in check of not eating too much of the wrong things.
The second trick is to learn to allow yourself to be hungry. Nothing will happen if you don't eat right when you feel hungry. In fact, drink a big glass of water to make sure it is hunger and not thirst. Then wait a bit, plan in your head what healthy snack you will eat once you have spent some time being hungry. This will help distinguish between cravings and real hunger.
Finally, an initial trick when starting a diet involves creating a plan for the first week or two and not changing it AT ALL. Eat exactly what you planned, with the exact number of calories you should eat and the times you should eat. This is when a personal trainer comes in handy to help figure out the number of calories needed and how to get the right calories.
Everyone has the ability to change their life. It is just getting all aspects of the process in line...the body and mind need to be ready and need to work for your benefit. Good luck!