There are a few strategies I use, depending on how the food is being served. If it is a buffet or potluck or picnic, or the type of event where a small amount of food is being served around as appetizers, it is easier. What I do in that case is make sure I go to the event having a pre-determined limit to what I'm going to eat & drink and how much, estimate as best as I can, and stop when I've reached my limit. I also try to select more healthy things (salads, fruit, nuts, grilled meats) and avoid bad things (sauces, fried food, sugary food, bread, soda & alcohol). And when I'm done, I walk as far away from the food as I can to avoid temptation and concentrate on socializing.
If it's a sit-down meal, it's a little harder for me, because I hate leaving food on my plate, and I don't want to be a bad guest. I try to let the host know to serve me small portions, and if that's not possible, I try to praise the food effusively and proclaim how full I am once I've had enough, instead of clearing my plate. Or, if I know that's going to be really hard to do, I try to work the rest of my eating plan around it. One meal isn't going to totally derail me anyway.
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"People often say that motivation doesn't last. Well, neither does bathing--that's why we recommend it daily." - Zig Ziglar