Create the outline for your story. Don't add a flashback in any story unless there is a place for it. Use the flashback technique in a way that will link the present with the past in a meaningful way. This should be achieved through not breaking up the continuity of the story as a whole.
Introduce a trigger into the story. A trigger should be a subtle way of introducing flashbacks into a story. Use them via an experience in the present that acts as a reminder of the past. Examples include a song, movie, or place. Senses such as smell, taste and touch can also help to transport the character back to an earlier time.
Use old letters written to one of the characters as a link to the past. This device should be used when a character is thinking about the past, especially about a past love affair. Rather than introduce letters by chance, introduce them as a way of further rekindling memories which are already stirring. Used sparingly, these memories will add to the interest of the reader, and should be used to explain some of the character's actions in the present.
Write about individual characters telling their own life stories. Describe the characters lives in a concise way. Brief life histories will be especially useful if they tell something unexpected about the characters.
Study your text, and make sure that the flashbacks you have used work successfully. Check that every flashback you have used does not come across to the reader as mere padding. Also check that the flashbacks you use do not confuse the reader by drifting in and out of the past and present in a vague way.