RECEIVING AND EATING ALMSFOOD
When receiving alms, a bhikkhu should:
be mindful to receive them appreciatively.
focus his attention on the alms bowl.
take bean curry only in proper proportion to the rice.accept no more food than will fill the bowl level to the bottom edge of the top rim.
When eating, a bhikkhu should:
be mindful to eat his food appreciatively.
focus his attention on the bowl.eat his food methodically, from one side of the bowl to the other.
eat bean curry only in proper proportion to the rice.refrain from taking food from the middle of the heap in his bowl.refrain from hiding his substantial food with rice, out of a hope of getting more.
refrain from looking at another bhikkhu's bowl intent on finding fault with him for not sharing his food.
refrain from making extra-large mouthfuls.
eat his rice in rounded mouthfuls.
refrain from opening his mouth until he has brought food to it.
refrain from putting his whole hand in his mouth.
refrain from speaking when there is so much food in his mouth that it affects his pronunciation.
refrain from lifting a large handful of food from his bowl and breaking off mouthfuls with the other hand.
refrain from nibbling bit by bit at his mouthfuls of food.
refrain from stuffing out his cheeks.
refrain from shaking food off his hands or scattering rice about.
refrain from sticking out his tongue or smacking his lips.
refrain from making a slurping noise.
refrain from licking his hands, his bowl or his lips.
refrain from accepting a water vessel with a hand soiled by food.
refrain from throwing away — in an inhabited area — bowl-rinsing water that has grains of rice in it.(Sk 31-36, 38-56)
from: http://www.accesstoinsight.org/tipitaka ... intro.html
There's more to it, regarding the amount of food a monk can accept (3 bowls that must be shared with the sangha), not keeping the offered food for the following day (so it must be shared), especially considerations for one is in wilderness, when food is left over among others.
The main points are not being greedy about food, never abuse the donor, never give the perception of taking what hasn't been given, not being attached to food of any particular kind and so on and so forth, as long as forbidden substances are avoided.