I decide backstory during play (not in a meta gaming sort of way).
It helps tie my character to the world more since I’m familiar with it after a few sessions, and the other players, plus the GM, tend to remember it more since I often give them a chance to get involved and improvise some details.
It pretty much amounts to mentioning that my character is from somewhere or did something once it gets brought up in a conversation.
I’ve found that since it always ties to things that get brought up during the game, it’s more likely that further down the line it ties into the adventure. There’s been multiple times that I’ve forgotten my own backstory but everyone else at the table remembers it and turns it into a key point of our campaigns.