Use grammatical transitions when you shift the story to the past. This can be easily done via the verb “to have.” For instance: “The last time Elizabeth ate dinner alone, she had been stuck in between flights at ...” The “she had been” alerts to readers that the story is now shifting into a flashback.
Use emotional transitions to shift the story to the past. Emotional transition means that the character is experiencing something that is triggering her flashback into the past. For instance, in Marcel Proust’s novel "Remembrance of Things Past," his character’s flashback is triggered by eating madeleines, a French cookie. Another example could include your character watching two young lovers being affectionate toward each other. This could trigger the character’s own memory of a youthful love affair. Determine the purpose of the flashback and its emotional center and find ways in which you can transition into it via present-day triggers.
Understand that flashbacks are useful when they answer questions about character and/or plot. For instance, a detective might come across a crime similar to one he investigated in the past and realize that the perpetrators might be the same person. The character can flash back to a previous interview he had with the perp. This flashback can provide insightful information about the investigator, the perp and the relationship that was forged between the two.
Use flashbacks to explain the background of your characters or your plot. For instance, a prostitute might have a flashback to her childhood that would explain where she came from, her relationships with her family and other details that would make her a fully fleshed-out character. You could also use the flashback to explain how she became what she is now.
Use flashbacks to drive a plot twist. For instance, the identity of a thief can be revealed in a flashback to the actual crime. This is especially effective in detective stories, but it can be used in other genres as well. Just make sure you provide enough clues along the way before you introduce the plot twist in the flashback. Otherwise, the plot twist and the flashback will feel tacked-on and inorganic.