When you had to write in school, you were probably placed under certain all-encompassing bans. “Never use the first-person” is a perennial favorite among teachers. I once had a respected professor who instructed students not to use semicolons. Now, the semicolon is a perfectly legitimate punctuation mark and has been put to many venerable uses. I believe my old professor banned it because so many people were prone to use it badly it was better that no one use it at all.
Many of the “Nevers” in writing are drawn up along a similar principle: Almost nobody does it right, so nobody should do it. Adjectives are an oft-targeted victim of this kind of reasoning. Another technique vulnerable to it is flashbacks, our topic of the day. Flashbacks possess a special nature, generally 
inclined to be awkward. They are written like narrative, but they are not narrative. Flashbacks disrupt the story, breaking up the flow and momentum of events to reprise old news. I have seen them done well, but I have also seen them done with extraordinary badness. Not all authors appreciate the nature or the purpose of the technique.
Two rules may be applied to the use of flashbacks. First, flashbacks must be relevant. A good way to think of this is that flashbacks must be revelatory of the story and not of the characters. What ought to be revealed of your characters can be revealed through the narrative proper: through their present talk, actions, thoughts. You might have constructed an entirely fascinating backstory, but you are telling a different story. Your story is in the present. The past throws light on the present, but the present throws light on people. There is no need of flashbacks to tell us about your characters.
Once I read a sci-fi novel that made excellent use of flashbacks. Very brief chapters, sprinkled among the narrative and set apart by italics, gave snapshots of the past. But these snapshots were keys to the story. They explained the nature of the present struggle, put forward mystery, foreshadowed the final revelation of villainy. Like all good flashbacks, they were dedicated to the story.
The second rule is that flashbacks must be brief. Again, flashbacks break the flow of the story, and for that reason they must be employed sparingly. Even if the flashbacks are genuinely interesting, people will become frustrated and impatient if the story is continually interrupted for field trips to the past. Flashbacks can spice up a story, but they should never be a main ingredient. Neither are they, in prodigious measure, likely to be altogether relevant. In the rare event that a prodigious measure is necessary, it is possible that you are starting the story in the wrong place.
The things people warn you about when you write fiction can generally be done. They just have to be done carefully. Flashbacks have been used to great effect, and you should always feel free to use them yourself. Only remember that they break the narrative and so must be very relevant and always brief.
