A simple prayer that is commonly used by Lutheran Christians before meals, according to the Wisconsin Evangelical Lutheran Synod, is: "Come, Lord Jesus, be our guest, and let these gifts to us be blessed. Amen." Another example from About.com for a prayer before meals is: "Bless us, oh Lord, and these thy gifts which we are about to receive from thy bounty, through Christ our Lord. Amen."
The following prayer may also be offered prior to a meal: "Father, as we gather to enjoy these gifts, fill us with the spirit of thanksgiving, so that we may offer you praise and thanks in sharing this meal. May you be blessed now and forever. Amen." Prayers before meals may also be customized to a specific holiday season. For example, a prayer at Christmas might say: "Father, the good news of our Savior's birth was announced first to the poor shepherds. As we give you thanks for the fruit of the earth, make us aware of the cries of those who are poor and in need. We ask this through Christ our Lord. Amen."
Overall, a prayer of grace prior to a meal should be heartfelt, thank God for providing the meal and ask God to bless the food. It can also give thanks for bringing the people together who are sharing the meal.