I don't think I discussed flashbacks here before and I couldn't find any entry when I just searched, so, since this came up on one of the AOL writers' boards, I figured I'd give my two cents on the issue here.
Way back when, I read in a how-to-write book that it was best to avoid flashbacks at all cost. That one should work into the current narrative any past event that is needed. Flashback was the device of last resort. Flashbacks, however, are fairly popular, and I enjoy writing them, especially as dream (or nightmare) sequences.
Someone suggested one type of flashback that works well is starting ahead (prologue or chapter 1, scene 1), then flashing back and writing the events that lead up to the opening scene. Someone else pointed out that is starting in media res, or in the middle. It's a good technique if you can pull it off, ie pick the right place to jump in and make the scenes leading back up to it worth reading.
But I don't consider that a flashback. Rather, I consider it telling the story out of sequence, or, in a bizarre way, it's almost as if the opening is a flashforward since it is just one scene, and the following scenes leading up to it are in regular sequence. And of course, writing out of sequence doesn't need to be just the opening scene. Joseph Heller played with the concept very effectively in Catch-22 and Joseph Wambaugh did it in one of his books, but I don't really recall which one offhand.
A number of books I've read recently tell stories in two parallel time frames, past and present. Some of course have been in different centuries with different characters, but Guy Burt, a British writer, plays with that kind of sequencing in The Hole and Sophie; both are psychological suspense and the technique works well in building tension as the reader is pulled in and out of the past where the major event happened to the present framing sequences. Mary Lawson did it too in Crow Lake, as the narrator in the present tells about her childhood, in alternating scenes/time periods.
I wouldn't consider any of these to be traditional flashbacks. To me, flashbacks are 1 or 2 scenes, maybe a few more, that break a chronological narrative to bring in a past event that is crucial for the reader to know at that point.
Even Dennis Lehane plays with sequence and narrative in both Shutter Island and Mystic River, tho not all of the past sequences are presented as actual scenes, but as memories and in a way, straddle the fence between writing out of order and using flashbacks.
And many flashbacks aren't necessary. The actual scene might not be all that interesting or can go on too long. What I often do is pull out bits of dialogue--from a fight, for ex--or a remembered image--from the past event and sprinkle that into the present as a memory. If the actual event, with all the detail, is necessary, and it won't work in a retelling, then I'll do a straight flashback. Which might be simply a scene with some framing device or a dream sequence, depending what will work in the story and for the character(s). It always come down, I think, to what the writer can make work. What I can write skillfully, someone else might not and I know other writers can do some types of scenes much better than I can.
I think traditional flashbacks lose their effectiveness if used too often, so I try to save them for things with some emotional impact and place them where I can get the most mileage out of them. I want them to really matter.