I've gone over this before, but it bears repeating: cultures have rituals to mark almost every important occasion: marking adulthood or marriage, for example. Or, it the case of the Minbari of Babylon 5, every single thing possible.
And then, of course, there are the rituals of death.
I've been thinking about this as something of a necessity for Banshee. Without going into spoilers, various aliens die, and treating their bodies in a respectful manner for their cultures is important to the survivors.
In coming up with rituals for the dead, be they for secondary-world human cultures, demi-human cultures, or alien cultures, you need to consider the purpose they serve. On some level, you should address three elements of purpose.
Practical: The body itself must be dealt with. Preserved, interred, cremated, otherwise kept from rotting on the floor. It can be as simple as burial, as complicated as mummification, or somewhere in between.
Spiritual: On some level, the ritual should consider the intangible element of the deceased, matching the faith of the culture. Does the lack of a proper burial mean the dead's spirit wanders the Earth, unable to reach the afterlife?
Emotional: Ultimately, rituals of the dead are for the bereaved. They are how grief is processed, either publicly or privately.
Ideally, your rituals address all three elements, or even better, integrate them into a unique whole.
Consider, for example, the mummification rituals for the kings of ancient Egypt. The actual mummification itself preserved the body, and it was interred in a tomb, designed for the purpose of giving the king everything he would need for the afterlife, allowing the people to process the death of a person they worshiped as a god, knowing that his time on earth was only part of his journey.
This is a pretty heavy-duty one, though. You might want to come up with something simpler for the average person.
How do your cultures deal with their dead?