Memories. Every three-dimensional character has them. But it's not always easy for writers to portray them. Even in some books that have made it to publication, the flashbacks seem forced and bring a reader outside of the story all together because of the artificial flow in the writing.
It's important to have a smooth ease of transaction between the present time of the story and the memories recalled by the character. And there's a very, very simple way to do this. I call it "Object-Action". Here's how it works:
Give your character an object in the present story. Make your character perform some sort of action on that object right before slipping into the flashback. Then, to come out of the flashback, make your character do another (or the same) action to the object.
For example. In the present scene, your main character is drinking a cup of tea. She remembers a a romantic moment. To bring her back to the present, have her take another sip of tea.
Perhaps your character is eating an apple. She flashes back to a dream she had the night before. She was an exotic princess. Bring her back to the present scene with the sound of the crunch as she chews the apple.
Or your character is at work. Bored and reading a newspaper. He easily slips into the memory of his glory days as a star football player. Want to come back to the present? Have him flip a page and realize he has no idea what he's just read.
Make sense? Let me know if you have any questions!!