Where are my London ancestors buried?

I've been hanging out on some genealogy forums recently, with a lot of people who are finding it very difficult to find their relatives' graves. And not just the burial spot itself, but even the cemetery in which they're buried. Here are a few tips if you're trying to find a grave yourself.

1. Cemeteries change names. This is the biggest cause of confusion. Nineteenth-century cemeteries are often named after the Burial Board which owned them; twentieth-century ones frequently after the London borough which managed them, and later, after the area in which they were situated. That means that cemeteries change names frequently.
2. Cemeteries share names. There are two "Hanwell Cemeteries", two "Paddington Cemeteries", several pairs of "Old" and "New" cemeteries... Check the burial date against the opening date of cemeteries to narrow it down.
3. Cemeteries have different names at the same time. Locals will frequently refer to [name of road] Cemetery, which isn't and never has been its official name. Check the map!
4. Parish cemeteries may be out of town. For example, residents of Islington are probably buried in "Islington Cemetery"... which is in East Finchley, nowhere near Islington.
5. Religion makes a difference. "Dissenters" - that is, Christians who didn't acknowledge the primacy of the Church of England - had their own cemeteries: probably Bunhill Fields until 1854; Abney Park from 1840. Christian denomination seems to have stopped being so problematic in the twentieth century.
6. The Jewish and Catholic communities had their own cemeteries. If you share the same faith as your ancestor, you probably know this - but if your family line changed or abandoned its faith, you may find your remoter relatives in a "specialist" cemetery.
7. Family still matters. Later in the twentieth century, many cemeteries have specific sections for the Chinese, Greek and Muslim communities, and other groups which are strongly represented in a particular part of town. Your family may well have chosen a cemetery further from their home in order to have a grave in one of these. Bahai people are probably in Southgate Cemetery. (Apologies to the national and religious groups I've left out; leave us a comment and tell us where your community likes to bury its dead, or scatter their ashes.)
8. Burial records don't necessarily exist. Deaths are a matter of public record. Burials and cremations are not. Many sets of burial records seem to have been lost during transfers of ownership of burial grounds. Others, happily, are being put online at Deceased Online, the central database for UK burials and cremations.
9. Don't expect a gravestone.I'm throwing this in here because of the number of people I've spoken to who've been devastated that their ancestor had no marker, or a marker that once existed has gone. Your relative may have been buried in a common grave; a stone that once existed may have been removed for safety (or grave reuse) purposes by the cemetery; there may never have been a stone in the first place (they're very expensive!). Before you go look, try to think that you're looking at the place itself, not solely an inscription on a stone.

If you're stuck tracking down a burial site, leave a comment with as much information as you have, and I'll do my best to help.