The first 1-3 bites of a food offer the most sensory satisfaction.  This isn't to say that you can't/shouldn't eat more than this, but it helps us understand why we sometimes overeat on foods that taste really good.  In clinical terms, we call this "hyperpalatability," meaning that the brain perceives certain foods to be more tasty than others.  Typically these are the foods that are higher in energy (i.e. calories), as this offered an advantage to our ancestors who had to hunt and gather their food.  It would have been hard to survive on only plants. 

Consider the scenario where you're out to dinner with friends and you order dessert.  Let's assume you're already full from the meal, but the sweetness of dessert sounds good.  You take that first bite and it lights up your tongue, and your brain thinks "Yum!"  If some was good, more must be better, right?  Not necessarily.

As you keep eating the dessert the amount of pleasure from each bite continues to diminish, so to achieve the "yum" factor you got in the first bite you have to keep eating more and more. 

If some was good, that doesn't mean more is better.

So how about eating the first 1-3 bites uber mindfully and deriving every ounce of pleasure out of it, and then leaving the rest behind?  Assuming you are full, this should be sufficient to satisfy your palate, and makes a nice end to the meal.