Dua during meals:

Dua Before meals:
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بِسْمِ اللَّهِ وَعَلَى بَرَكَةِ اللَّهِ
* In the name of Allah and with the blessings of Allah I begin (eating)".
* 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.
[Abu Dawud 3/347]
[At-Tirmithi 4/288]
[Al- Albani's Sahih At-Tirmithi 2/167]

Dua After meals:
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الْحَمْدُ لِلَّهِ الَّذِي أَطْعَمَنَا وَسَقَانَا وَجَعَلَنَا مِنَ الْمُسْلِمِينَ
* "All praise belongs to Allah, who fed us and quenched our thirst and made us Muslims".
اللَّهُمَّ بَارِكْ لَنَا فِيهِ وَأَطْعِمْنَا خَيْرَاً مِنْهُ
* "O Allah, You grant us blessings in it and grant us better than it".
الْحَمْدُ لِلَّهِ الَّذِي أَطْعَمَنِي هَذَا الطَّعَامَ وَرَزَقَنِيهِ مِنْ غَيْرِ حَوْلٍ مِنِّي وَلَا قُوَّةٍ
* "All Praise belongs to Allah who granted this meal, (which I received) without my power or effort".
All previous sins are forgiven if this dua is recited after meals.

[Mishkat, Kitabul-li-baas]
[Tirmithi, Abu Dawud, and Ibn Majah. See also Al-Albani, Sahih At-Tirmithi 3/159.]