Placement of flashbacks is important. Don't put them in an action or fight scene as it will slow things down for your reader, and you risk the reader skimming ahead to where the action picks up steam again. On the converse, a flashback can liven up a static scene where you have the character alone and she has time to think. The place for flashbacks in a story is mostly likely nearer the beginning or middle and not the end.
Be sure you have a natural trigger into a flashback and you'll be less likely to create an information dump for your reader. Pay attention to where the flashback ends and the character is returned to the present. Make sure you delineate both the entrance and exit points of the flashback.
Some say no flashbacks at all, yet sometimes they can offer motivation to your character if you write them well and it illuminates where the character faced a similar situation. They need to be justifiable and not just a flight of fancy for the writer.
As with all elements of writing, there is an art to what will work for your story.