Make sure a flashback follows a strong scene. Flashbacks can be problematic in that they remove your character and therefore your reader from the action in your narrative. A strong preceding scene can ensure that the narrative is sustained.
Ensure your reader knows exactly where and when he is. Make the transition into the past clear.
Use the correct verb tense. If your main narrative is written in past tense, then the first sentences of the flashback should be in past perfect. You can then continue in simple past.
When the flashback is over, make sure the transition to the ‘present’ of the narrative is smooth and clear, so that your reader isn’t confused or disorientated.
Acknowledge the flashback. It should have an effect on the character who experienced it and on the narrative.