Why do you eat that first bite? If it's because of hunger, choose something healthy and filling like an apple. Make a list of healthy choices and post it on the fridge, cupboard or wherever you keep the chocolate or other things you usually snack on to help you think about other, better choices. (remember the commercial where the person realizes, "I could have had a V-8?
If you are eating for some other reason, what is it? Boredom? Loneliness? Sleepiness? Craving for salt, sugar, etc? What are you doing while you are eating? If you are watching TV, on the computer, etc., maybe doing something more active or something that occupies your hands will help you not get started. I used to read while eating, and realized I liked reading so much I was eating more to give myself permission to read longer.
Do something that keeps your hands too busy to keep eating - knit, crochet, woodworking, etc (a "messy" hobby is a bonus - hands covered with paint, varnish, clay etc aren't so tempting to eat with) ;-).
Lastly, consider going to bed earlier. The extra sleep might be a bonus and I've noticed I tend to snack more when I'm trying to stay awake