Gymbob, I'm giggling at the thought of your mother sneaking out in makeup!
I have 4 DDs, the eldest is 15. We had huge issues about clothes around Year 9 time. The typical series of events was that we would go shopping, she'd point out a tiny scrap of fabric dress that she wanted, I'd say no, she would sulk. We'd then compromise on something I thought was ok. I would feel smug. She'd go to a party/friends house/out. She would swap into an even tinier scrap of fabric another dress borrowed from a friend with a 'cool mum'.
I didn't find out for a while, but a friend of mine saw her facebook. Cue huge row.
To cut a long story short, after she was grounded for a seriously long time, we had a chat and decided some ground rules. She now has stuff that I cringe a little bit when she wears, but it's better than what she was wearing.
She's allowed short dresses, but not ones that involve her boobs being out as well. This is ok. This is not.
Similarly, she knows if it's skintight it's more likely to be vetoed than a more floaty style. She can wear a boob tube top or a cropped top with say, rolled up chinos or a knee length skirt, but not a body con mini skirt or hotpants. She can wear the body con mini with a loose t shirt or a blouse.
I know she's a bit older than your DD, but I have followed these rules with my younger DDs. DD3 is in Year 7, and she knows she can wear, say, denim shorts with a t-shirt or a hoodie, but not with a cropped t-shirt. She can wear the cropped t-shirt with chinos.
The thing I tell myself when DD1 looks more risque than I'd like is that they're kids. No matter how 'sexy' they try to dress, they're skinny, angular, flat chested, kids. They go to kid events with other kids. IF they were going out into town to a night club, I'd be a lot more concerned. But if DD1 puts on a short dress to go to her friends house to see other 15/16 year olds, it's ok.
I try to keep strict on school clothes (although I'm pretty certain all 3 of my eldest DDs whack makeup on and roll the skirts up on the bus), or if they're going to see Granny, etc etc. If they're just knocking around with other kids, and I know they're safe/supervised, I'll let it slide a bit.
I appear to have written an essay. It's bloody hard work having teen/preteen DDs isn't it? 
anyone?