Headhunting! An iconic practice in Irish heroic myth, also one readily apparent on the continent. The severed head was something of a significant spiritual symbol to the Celts—the statuary at Entremont near Aix-en-Provence leaps to mind. Propitiously there exists a written account from Diodorus Siculus of the Gallic Celts that cuts close to the quick of your inquiry:

When two armies are drawn up for battle, it is their custom to step before the front line and challenge the best of their opponents to single combat while they brandish their weapons in front of them to intimidate the enemy. When an opponent accepts their challenge, they recite the brave deeds of both their ancestors and themselves, at the same time mocking the enemy and attempting to rob him of his fighting spirit. They decapitate their slain enemies and attach the heads to their horses' necks. The blood-soaked booty they hand over to their attendants, while they sing a song of victory. The choicest spoils they nail to the walls of their houses just like hunting trophies from wild beasts. They preserve the heads of their most distinguished enemies in cedar oil and store them carefully in chests. These they proudly display to visitors, saying that for this head one of his ancestors, or his father, or he himself refused a large offer of money. It is said that some proud owners have not accepted for a head an equal weight in gold, a barbarous sort of magnanimity. For selling the proof of one's valour is ignoble, but to continue hostility against the dead is bestial.^^1

Cedar Oil would allow for some preservation both in the microbial and aromatic sense. Though as Diodorus notes it would only be the most hallowed trophies that might be so conserved. In early Irish literature heads are often simply hung streaming gore off a war-chariot, or as here in the Táin Bó Cúailnge where Cú Chulainn stakes heads left to rot as a warning to the advancing host of Medb:

There he cut out a tree-fork with a single stroke of his sword and stuck it in the middle of the stream, so that a chariot would have no room to pass it on either side. (It is from this that the name Ath Gabla comes, the ford of the forked branch.) The warriors Err and Innel, and their two charioteers Foich and Fochlam, came upon him. He cut off their fours heads and tossed them onto the four points of the tree-fork.^^2

However if sufficiently illustrious, the elevation of the skull to sacra could become decidedly elaborate, as Livy recounts of the defeat of the Roman Consul Postumus by the Boii:

Postumius died fighting with all his strength trying to avoid capture. The spoils stripped from his corpse and the severed head of the commander were taken by the Boii to their holiest temple. Then, after they removed the flesh from the head, they adorned the skull with gold, according to their custom. They used it as a sacred vessel to give libations on holy days and their priests and the custodians of their temple used it as a goblet.^^3

Analogues are noted in both Herodotus' Histories and the Shǐjì of Sima Qian—so I've little doubt there are further methods from other regions.

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^^1 ^(John Koch and John Carey. The Celtic Heroic Age. Literary Sources for Ancient Celtic Europe and Early Ireland and Wales. s.v. "Diodorus Siculus". Aberystwyth, Wales: Centre for Advanced Welsh and Celtic Studies, 2009. p. 12-13 ^¶29)

^^2 ^(Thomas Kinsella. The Táin. Dublin, Ireland: The Dolmen Press, 1969. p. 273)

^^3 ^(John Koch and John Carey. The Celtic Heroic Age. Literary Sources for Ancient Celtic Europe and Early Ireland and Wales. s.v. "Livy". Aberystwyth, Wales: Centre for Advanced Welsh and Celtic Studies, 2009. p. 36-37 ^¶23.24)