I make a point of being thankful for for my food, for several reasons.
1. It is good not to take good things for granted. Gratitude makes for a richer and happier life, and I've found it actually enhances compassion for people who don't have as much good fortune as I do. I also enjoy my mealtime companions more.
2. It also enhances my appreciation for our earth and its creator, the hard work that went into producing the food, and the animal or plant whose life was ended to feed me. I'm less inclined to be wasteful.
3. Blessing my food puts me more in the present moment, and I notice and enjoy my mealtime experience more. I'm less likely to eat absentmindedly, and have a much easier time noticing and stopping when I've had enough.
4. I've found that when I bless my food, I seem to digest it more comfortably and I'm less likely to have an allergic response to it (I have many allergies).
My daughter and I said a simple, unobtrusive grace when she was little, and now she has taught her son (and hubby) to do the same. My husband and I still hold hands for a moment of silent appreciation. We all do this both at home and in public, unless we sense that it would make other dinner companions uncomfortable. Then we just give our own silent blessings individually, or hold hands under the table if we can reach, and smile at each other before starting to eat.