Tashi Delek,
is there a certain prayer you should say before the meal? I don't know it so what I usually do is to thank to everybody who contributed to making this food and I ask for the blessing that eating this food would help me sustain my body for the reason so I could continue my way on Bodhicita path. then I recite the Mantra of Chenrezig for a couple of times. And (as I learned on the seminar - thank you, AniChodron!!) I always leave some pieces of the food for Pretas, so afterwards I offer it to them.
I will be happy to hear your ways of doing this.