The sacrificial offering to the gods and the subsequent meal that all participated in represent both a recognition of divine power and an attempt to align one's life with divine will. It was an act of humility carried out in the hope that the gods would respond favorably and there was great disappointment if the act was not successful.
The sacrifices usually consisted of humans, animals and any object such as a shield or sword that was expensive and highly valued. These included any objects promised to the god(s) in the hope of victory such as the treasure of the opposing army.
As a rule weapons were offered to the martial gods such as Odin, Tyr and even Thor. Farming and agricultural tools, on the other hand, to the fertility gods, i.e. the Vanir.
Human sacrifice, with very few exceptions, was made to Odin. Offered up to him were prisoners-of-war, servants or criminals. In times of crisis members of the most noble families could be sacrificed to appease Odin and there is even a case in Scandinavia of a king being sacrificed after a few years of bad harvests.
The Greek historian Strabo had given us a detailed account of human sacrifice in his report on the Cimbrians: "The women who had accompanied their men into battle were led by priestesses who could predict the future. These priestesses were gray-haired and clad in white garments. An overgarment made of linen was fastened with a pin at the shoulder. They also wore a belt of bronze and went barefoot. With a sword in their hands they walked towards the POWs, placed a wreath on their heads and led them to a 20-gallon bronze pot. They climbed up a short ladder leaning against the cauldron and slit the throat of each POW who was lifted up for them. They prophesied the future based on the manner in which the blood flowed into the pot. Other priestesses slit open the bodies of the POWs and after examining the position of the entrails proclaimed that the Cimbrians would win victory."
After this victory the Cimbrians did indeed sacrifice all POWs to Odin by hanging them from trees and when they themselves were vanquished in the battle of Vercellae in 101 B.C.E. they even hung themselves from trees, wagon axles and the horns of cattle as a sacrifice to Odin and the hope of being accepted into Valhalla.
After destroying the Roman army of Varus in 9 C.E. the Cheruskans sacrificed the officers on altars and hung the soldiers and horses's heads trees. The Hammars stone which was erected around 700 C.E. on the island of Gotland is the only picture that remains of an actual sacrifice. The priest on the left has placed his hand on a sacred rock and may be uttering a prayer, while the priest on the right is preparing the kill the victim in the middle with a spear, Odin's weapon. To the far left of the picture is a warrior hanging from a tree which is bent by his weight. Presumably he will die when the tree is released. To the right of the picture are four men, presumably members of the community. One hands a bird of prey to the priests, the others stand at attention with swords point upwards. Over the altar are good-luck charms such as the eagle, the raven the Hrungnir heart consisting of three interlocking triangles as desribed in the Poetic Edda.
The most frequent animal sacrifices were horses, cattle, goats, sheep and pigs, but also birds from time to time. They were generally male animals and sacrificed to the gods with whom they were associated: horses for Odin and Freyr, goats for Thor, pigs for Freyr and Freyja-Carmen.
The ritualistic slaughter of the sacrifices followed an age-old pattern: the animals were strangled and then held over a bronze pot where their throats were slit. The priests examined the flow of the blood to determine the will of the gods and then called out the results to the crowd. After the blood ceased flowing, the animals were cut open and the position of the entrails examined. The flesh was then taken carefully off the bones (the bones could not be broken so that the animals could be resurrected in the World of the Dead or Other World). The meat was placed in a pile before the deity usually symbolised by a stick-figure. The head was nailed to a tree or, if no trees were available, to a sacred stake. Occasionally the bones were tied together and also hung from the stake. The meat was used for the ensuing feast and in some instances the brokens of certain animals were broken in order to provide marrow for the broth, but even these bones were then collected and offered up to the stick-figure. Excavations at Oberdorla and in Scandinavian indicate that there was no difference in the sacrificial customs among the different Germanic tribes.
The inedible parts of the animal were burned on the animal and thus also sacrificed to the god. Before the meat was cooked in the vats the priests dipped fronds into the blood and springled these over the gathered crowd, the altars and other sacred objects. After temples were erected to compete with Christianity they also springled the statues of the gods and the walls of the temple with this blood. This blood was a means of establishing A holy communion between the gods and the worshippers. The same applies to the feast that followed. It was a very solemn religious event when the divinity and the community are one.
The feast started with a bowl of broth and then the meat was distributed. After this a drinking horn filled with mead, the drink of the gods, was passed around after having been blessed by the priest using traditional formulas and stressing the need of the particular occasion, i.e. need for a good harvestm gratitude for a victory, etc. The horn was passed in the direction of the solar path, i.e. from east to west. Going against the sun's path was thought to bring disaster.
The feast had to be sumptuous to honor the deity and show respect for the Germanic sense of hospitality. It also symbolized fertility. Nothing was supposed to be left over, but if it was, it was taken and buried or sunk in a lake or river so that it could continue to serve its purpose.
The state of intoxication achieved at these sacred feasts was regarded as spiritual reunification with the deity. Other cult activities serving this goal are praying, singing, dancing, playing music and perhaps even performing a ritual drama.
http://www.public.asu.edu/~atrja/sacrifices.html