There are a few questions you need to ask before you can decide on a culture's funeral practices in a fantasy world with undead.
1)
Why do the dead rise?
There's several possibilities here, and each one would affect funeral rites.
Pervasive negative energy, aka Corruption/Taint - In this scenario, any bodies left in a specfic place that is tainted with evil/negative energy will eventually rise as an undead. One solution: just make sure no one is buried there.
Another is to cleanse the location of taint or, if the taint was within the person themselves, bless the body.
A fey curse - Perhaps the people were cursed by fey for some percieved wrong. In which case, the people may believe that lining the coffin/burial chamber with a thin layer of cold iron would either prevent the curse, or at least prevent the undead from breaking free. A
remove curse spell on the body might also be appropriate, and works for curses from non-fey sources, and other materials might be used for other kinds of curses (garlic stuffed in the mouth, for instance).
Disease - The old
Resident Evil staple. With spells like
cure disease, it's not so much of an issue. The spell would just be worked into the society's rituals.
Necromancy - This one is harder to pin down. Someone is intentionally causing the dead to rise. In this case, strengthened tombs, deep burials or chasms might be used to keep the bodies away from being used, as well as preventing undead from getting out.
2)
How do you put down the undead?
This can vary from setting to setting. However, if it's consistent, a culture would take advantage of it in their funeral rites.
Beheading / shot to the brain - If decapitating an undead will suffice to kill it, you could borrow from an old European practice and simply cut the head off the body before it's buried. If damage to the brain is necessary, well, there's always a nice big metal spike...
Sunlight - Curse of vampires everywhere. In which case, an open-glass mausoleum would be appropriate. Especially if a magic item that duplicates sunlight could be cast on a lamp on the ceiling, for nighttime.
Silver/cold iron/wood/etc. - If a specific material harms the undead, use that material in the funeral rites. Drive a wooden stake through the heart, sprinkle silver shavings into water and inject it into the body, place a cold iron holy symbol on the forehead, etc.
Contingency spells - High level magic, but you could combine
contingency with a spell designed to destroy undead the moment the body starts to arise.
Fire/acid - Old standbys. Cremation or dissolution of the body is a good way of preventing corporeal undead from rising, though it may make things worse for incorporeal undead hauntings. A good
disintegrate spell works, too.
Dismemberment - Scatter the body parts to the four winds, and you won't have as much trouble. Except for those pesky crawling hands...
Another solution in this vein is cannibalism. I believe this was brought up in the
Midnight campaign setting, where cannibalism in a society both helped alleviate famine and prevented the bodies from arising.
3)
What does the culture believe happens to the dead?
All of those options can be modified by how the culture believes the soul is treated upon death.
The End - If a soul is simply gone upon death, then the undead are nothing more than magically powered corpses. The body can be destroyed in whatever manner necessary.
Ascencion/Descencion - The soul either goes to an afterlife of reward or punishment. Either way, it doesn't come back to the body. Again, the body is just a body.
Hanging around - Sometimes, the spirits of the dead stick around to check up on us. If they aren't happy, they can do some nasty things to the living. Follow the cue of any haunting movie you want, or you could go for a more Eastern flavor of ancestor "worship." The deceased must be buried in the proper ceremony, and placated in rituals on occasion, or they might start bothering the living. In this case, the body isn't as important as the funeral ritual itself.
Return - Some cultures might beleive that one day the righteous will be returned to their bodies, and the wicked destroyed. In which case, damaging the bodies themselves would be anathema! For these cultures, you'd have to use something that prevents undead from rising (like the materials that stop undead), without actually harming the body or hindering its eventual resurrection.
Contained - The soul remains tied to its body and, if corrupted or angered, can cause the body to rise up. To prevent undeath, without angering the soul, the body would have to be preserved and restrained. Mummification might suffice, or sealing within enforced tombs. Binding rituals might also be appropriate, preventing the body from ever leaving the burial location on its own (vampire's native soil, for instance).
Possession - Other spirits can possess the corpse, and use it to cause harm. Depending on the culture, it might be most appropriate to destroy the corpse, or to simply place it somewhere it cannot cause harm. Otherwise, rituals & magic to keep spirits out of the burial location would be good.
