American Beauty, told using flashbacks, won the Oscar.
Greenlight's formatting is correct -- it's preferred that the word FLASBHACK be put in the slug line for clarity.
Scripter1's list is excellent. I'd also ask, is there something that the flashback gives you very specifically, story-wise, which cannot be gotten through straight chronological time? Think of The Usual Suspects, which has a narrator and then shows the majority of the story in flashback. Without that narrator telling the story (an unreliable narrator), you wouldn't have the same story, the same surprise. It would end up being a straight-up crime or detective story, and instead, there is a psychological layering which gives the story texture and tone and something extra than otherwise simple double-cross kind of story would be able to give.
I think the main mistake people make when they're using flashbacks in screenplays is that they want to start the story in medias res, so they start it in the present, with some action, but then they've got all of this backstory and they're trying to figure out a way to get the audience up to speed, so they stop the forward flow of the story and show the backstory and then catch back up to where the story started. While it can, technically work, it's not going to get past a lot of Hollywood readers unless the writing is stunning.
Stephanie, you're not going to want to hear this, but the problem I have with your story is that the action is somewhat static. Your main character moves through memories and meets people and has an epiphany or a series of epiphanies, and then the time is running out, she's got to get to her mom's side to have the reunion / moment before her mother dies. Two of my best friends read for the Nicholl and a third was a reader for a studio for years (and I was in the business for ten, have optioned scripts, a zillion meetings, the film was supposed to go into production... in New Orleans. No idea if it's going to happen now.) Anyway, they see these kinds of stories all the time. Ya Ya sisterhood is a good example of one done pretty well -- the book sold tons, obviously, and the movie did okay, but not stellar box office. You'll have to look at those sorts of stories as well as stories which use flashback and ask yourself what is the hook that's going to make this story unique? Is there an angle, a tone, a slant on the story that's different than the Ya Ya story? (The impending death of the mother makes it a Lifetime movie, unless you figure out a bigger hook.)
I honestly don't mean this to sound harsh. I wrote a script like what you're talking about, and it got me an agent. It got me a zillion meetings, which is a great thing. But I got really tired of banging my head against the wall hearing how those sorts of scripts don't sell. (It's not the use of flashback that's going to kill you, but it can hurt his story.) Flashback ends up making that story a passive journey for the main character. They aren't facing real obstacles except from within and reorganizing their perception of themselves, which feels static to a filmmaker.
I think it can be done. I think you've got the start of a very good idea. I'd push you to think more outside the box than you have as yet.
-toni
MFA in screenwriting, ran what the WGAw called one of the "top 3 sites" for screenwriters on the internet... probably full of crap, but trying to help