Flashbacks. They can be a wonderful way to bring in some back story, or illuminate a character's motivation or reasons for reacting a certain way. But I need to rant just a teensy bit...
Flashbacks should not need to be announced. Please. It's disruptive, and drags me right out of the flow of a story. I'm reading along happily, and there it is. The clue-by-four, in all caps and bold lettering. FLASHBACK. Now I've come to a full stop, and I'm yanked right out of my immersion in the story, and that's not fun at all.
If you've been using a regular font, italics can serve to highlight a flashback. Really. It's how it's generally handled in the books you buy in bookstores. Or set it off another way, if you've already reserved italics for characters talking to themselves and bold lettering for mental telepathy. But please... no great big warnings that we are entering the flashback zone.