At a mitzvah-meal be careful to not lose your merit [from the meal] by speaking forbidden things, such as mockery, hurtful words, frivolity and light-headedness and the like. Rather, always have the fear of Heaven upon yourself, and then, when you behave properly, in accord with the Torah, your table will be considered like an atoning Altar.
Recite Birkas Hamazon (Grace After Meals) slowly, out loud, and with joy — because when you pay attention to what you are saying in Birkas Hamazon and you think deeply about the words, it is impossible that you won’t be happy. From the holy Zohar it is apparent that it is extremely important and there is an obligation upon the person to do whatever he can to recite Birkas Hamazon with joy.
A method to achieve focus while reciting Birkas Hamazon is to recite the blessings with one’s eyes closed or looking down.
The author of Sefer Hachinuch writes that he has a tradition that whoever is meticulous in his reciting Birkas Hamazon receives his livelihood with honor his whole life. Now, the extent of the reward shows how much pleasure this matter gives to one’s Creator. May all one’s actions be for the sake of Heaven.