I’m with Christopher Eccleston on this one: when I stop playing a character, I never go back to playing it, even if it would be for a 50th anniversary celebration.
Usually, when me and my group end a campaign, we start up something else entirely. For example, now I’m playing Vampire. If we go back to D&D we’ll probably start at 1st level, so previous characters are often too experienced. Also, there are many different character concepts, and returning to the same character means I can't play them.
I do use favorite previous PCs as NPC in new campaigns regularly. Sometimes they’re given a new name, appearance, and role so the players don’t recognize him or her. At other times, the character is pretty much the same. I like to have some continuity between my campaign worlds, even if they're not really in the same universe. For example, Quinzel is the character I played in my brother's Greyhawk campaign, Saldoran Gorst is from my Dromenon campaign, and Kata from an old 2nd Edition campaign all exist in the Weird Opera world in one form of the other.