i think az is absolutely right ~ write it first as fat as it wants to be and then edit/revise. i strongly advise having an editing technique that works for you, though. it's hard to answer the question in anything other than general scenarios for me. in your case, i'd drop some backstory in gradually like clues that leads up to the 'big revelation' lest it come off as some out-of-left-field idea. and as maestro said, keep what you need, which is really the trick, isn't it? you can write the backstory as notes on the side or as part of the story, i think it depends on what way works best for you. for me, my characters either start off as being pretty obvious or somewhat with an untold past which gets trickled out as the story goes along. i don't do family trees or character sheets or any of that. then again, i don't work from outlines, either.
probably one reason why i don't get into young characters is because they have no past, nothing interesting has happened to them. or their past is so ridiculously abusive it's hard to imagine them ever being truly salvaged by the end of the story, having resolved witnessing their father killing the rest of the family, growing up a sex slave in the bordellos and having to kill people in the dark for food money, then, at 16 years of age, the end of the story comes along and true love has healed all the wounds. pretty cheesy, eh? lol.
a lot of backstory doesn't need to be said anyway: for me, ideal backstory is mostly inferences, maybe backed up with a couple of words. i think the overall riding theory for me is to use the past only if it relates to the current affairs, try to tie it in the story as characterization and/or plot resolution, and let the reader fill in their own backstory for the most part given the clues there. then try to hide the backstory as i would try to hide the exposition when i could. i mean, when you see a character popping medication like pez candy, it's a sure bet that somewhere down the line we'll find out why, no? as long as it's got something to do with the story. characters with scars, physical or emotional, that gets some early play are likewise bound to be elaborated on. subtle, less meaningful explanations such as how the MC had been trained to be a mediator through her family situation i think might just be okay to get out of the way in as few words as possible at the end of a sentence as flat-out background. it's not a sin or anything, and by way of being economic with words and getting straight to the point, i think it's sometimes better than winding dialogue in an attempt to hide it, imo. it's rather very situational for me. i mean, if she's a middle child, i think that's pertinent information eventho it really might not come into play anywhere else. if she's deathly afraid of heights, say, and there's a scene involving her needing to be high up (pretty cliche, i grant you), then perhaps even a flashback is warranted (i personally try to avoid writing flashbacks, but i'm not against them, either).
a lot of times what's in the backstory are reasons why the character is the way they are today. often the backstory is the impetus for character growth (overcoming a fear or past trauma). there are a lot of pretty obvious frameworks when that's the case, but i think what you're asking about are those little details what have naught much to do with the story, per se, but are still worth mentioning. i say there are times when you can simply mention one and get it over with, or you can mention it in some dialogue, or you can have her rifle through her purse, discribing the contents (a bottle of pills, a pack of condoms, a picture of her with her siblings, i wad of attorney business cards, a few .22 bullets, twisted engagement ring minus the two carot (karat? i always get those confused ~ i just know one is for gold, the other is for gems) diamond, and a new york state driver's license ~ okay, that's pretty cliche, eh? lol), or some other little device that paints the character the way you want him painted at that specific time in the book.
i used to think more about this stuff than i do now, lol. i'm sure there are lots of things that i do now as rote, but those are some of the options i'd consider now. that is if i actually thought about it which i probably should, lol.