Avoid flashbacks if at all possible. A story should begin at the beginning and go to the end with few visits to the past. However, our characters do have a past, and their emotions tie into their past, happy or sad. Therefore, if the information is necessary for your reader to understand your story then go for it. But be careful.
The best way to use a flashback is through dialogue, action and/or conflict. If you don’t want to put your reader to sleep, narrative is a big no-no. So make your flashback vivid with plenty of tension. “Stein on Writing” says it better than anyone. “Fiction should seem to be happening right now.” This is an important statement because a flashback can be obtrusive and stop the story dead. Before using that flashback, ask yourself if your story would read better without one.