Sit properly, your body should be facing the table squarely. Elbows out, legs open not crossed. Your body should be open, ready to accept the goodness. Lean forward on every bite you do. Use both hands if it's eaten with bare hands, if it's eaten with utensils use spoon and fork. Eat the food while it's smoking hot, make sure you blow air to cool the food you're about to put into your mouth. Do not over fill your mouth. Smile after you swallow the first mouthful, and say your compliments. Short and simple praises is enough. Do not compare your wife's cooking to your mothers if your mother is a better cook than your wife.
Do not say it's salty, or too sweet, or the taste is off. She already knows it, you don't need to give her feedback now. Feedbacks can be given after the plates are cleaned.
Don't ask any questions on how she cooked it. She knows you don't mean asking questions. You will not remember the recipe even if she tells you how it was done.
Eating a lot is the best compliment ever.