Another tradition in One Piece I noticed is at least one flashback in every story arc. At least the Major ones; I mean, look:
Beginning (Romance Dawn, Alvida, and Morgan): Luffy's flashback and Zoro's flashback.
Buggy: Buggy/Shanks flashback, and ChouChou's flashback
Syrup village: Usopp's flashback, and Gaimon's (Ok, he's not on Usopp's island, but he appear's right before it!)
Baratie: Sanji's Flashback.
Arlong Park: Nami's Flashback.
Lougetown/Laboon/Whiskey Peak: Laboon's Flashback, and I think Smoker had one…
Little Garden: Giant's Flashback.
Jaya: Something about those monkey guys and Cricket...
Davy Back Fight: Not so sure about this one...
Water 7/Enies Lobby: Franky/Iceburg one, and Robin.