Prophet (SAW)'s Prayers Before meals

Recite:

بِسْمِ اللَّهِ وَعَلَى بَرَكَةِ اللَّهِ

In the name of Allah and with the blessings of Allah I begin (eating)". (Mustadrak)

On forgetting to recite Bismillah at the beginning, recite:

بِسْمِ اللَّهِ أَوَّلِهِ وَآخِرِهِ

"In the name of Allah in the beginning and end".

By not reciting Bismillah, Satan has an opportunity of partake in the food. (Mishkat)