If you want to do a prayer, there is one commanded in Deuteronomy 8:10. It applies to Canaan, but I think it can still be interpreted as a legitimate blessing after a meal no matter where you are.
So it seems that we are commanded to say a blessing after a meal, not before. And I think this is smart because a person is far more likely to be in a good mood for a blessing after a meal rather than before. Right?