Flashbacks are difficult. Frankly, you should avoid using them whenever possible. Seriously. The difficulty of a flashback is that while it may inform your character, you’re not moving forward. Best to keep them brief and sprinkle them throughout your book as needed.
Here are some thoughts:
Use minimal, but vivid detail. If you choose your words carefully, you won’t have to use many of them.
Insert “triggers” to your character’s memories. Your character hears a song or sees an image that makes her think of something else – the more unique or unexpected, the better. These images bring her back, but maybe for only a sentence or two. And it can’t happen constantly, or else you risk being too stuck inside your character’s thoughts.
Avoid “it seems like just yesterday” and “I remember when” and anything else that sounds like an obvious opening to a memory. You might laugh, but I’ve made this mistake – it’s just lazy writing. There are more creative ways to embed memories and flashbacks (important ones) into the text without taking your character away from the immediate action for too long. That’s the goal – share what you need to share, but don’t remove us from the action for so long that we forget what’s going on, or lose interest.