Post History

Source: https://writers.stackexchange.com/a/46847
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/46847
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

I am not aware of a "common practice" - writers are fickle beasts who tend to disregard rules. But there's nothing that says the progression of your story needs to be "linear unless marked otherwise".

The first example that comes to my mind is _The Dispossessed_ by Ursula Le Guin. First chapter of the novel - the MC makes a dramatic escape from his homeland to another country. Second chapter - the MC is a very young child. The chapter is not in italics or anything like that. It's clear that the progression from chapter 1 to chapter 2 isn't linear, pursuant to the fact that one doesn't jump from being an adult to being a baby. Readers are smart, they can figure it out.  
From there, the tale proceeds on two parallel timelines: one explores the MC's life after leaving his homeland, the other - his life before. The character's age and the entirely different situation provide the cues the reader needs to position themselves.

Making time-line jumps chapter-breaks is useful: it's a useful way to mark a transition for the reader. Especially once you've established the fact that such transitions occur, and there may be little else to mark it. It's also useful to establish early on that flashbacks are something that happens in your story, that is that it's not all going to be linear. In the aforementioned example, Ursula Le Guin sets it by jumping to a point where the MC is a child. This makes the transition obvious. Later on, when the MC is an adult in both timelines, she relies on the fact that the transitioning back and forth has already been established.

In addition to jumping to a sufficiently-distant point in the past, you can establish that "we're not where we were a moment ago" through switching to a different POV, through recounting it in present tense (though that has some additional effects on how the reader would perceive the passage), or through otherwise making a significant change from what has just been established. (For example, if main story-time is in summer, your flashback can be into winter.)

Original score: 1