If there’s one thing that freaks out new writers is how to write the flashback. So many times on the forums they’re told not to write them; to avoid them at all costs. Sad thing is you need to write them sometimes so let’s briefly discuss them.
One thing to avoid is putting them as the opening scene of a novel. That is boring to an agent and that’s who you got to impress first.
From there on, don’t make it difficult to merge it into your main story. Mine are seamless in the Talia novel and are then used to make a mini-scene within a scene and then I merge it back into the main story. A couple times I start a chapter off with them and one time I have an entire chapter that is one that I use to tie the loose ends up between Talia (who is nearly immortal) and her son, who is mortal.
To put it in a nutshell, don’t tie it all up in a single way and stay away from the cliché dream sequence either. There’s tons of ways to add it to your book! Explore them all and see which one fits your style and story.
I think it is better to have your character see something that is like a memento. Maybe he or she see something familiar and starts to think about his or her past briefly. That way you can stay on the main plot without going off track.