I'm 35 (and I only feel old when I talk to my tweenage niece, who thinks I am just ANCIENT

), and primary concession I've made to trying not to dress "too young" is that I used to be kind of a goth/punk/alternative chick in my late teens and early- to mid-twenties (blue-black hair, all black clothing, fishnets, Doc Martens, very Janeane Garofalo), but not only is that sort of not really a thing anymore, it also seemed kind of "trying too hard" as I finished school and had to get a real grown-up job.
Like others said, I find Old Navy a great and inexpensive option for lots of things, both staples (dress pants for work, basic jeans, tank tops... I LIVE for their tank tops) and whatever little trend is going around (without making a huge cash outlay for said trends). I also get blazers for work from inexpensive catalogues so that they're a little heartier in terms of care than something from a slightly higher-end place that is usually dry-clean only.
I'm at a stage in my life and a career path where I do occasionally need formal gowns (I swear by Bluefly) but also lots of shorts and tee shirts (I get a long summer vacation and NEVER wear business suits if I'm not at work). I have a great mix of high-quality stuff and super cheap, almost disposable stuff. I'm also a big nerd and have a bunch of goofy tee shirts celebrating my favorite science fiction shows, and I wear those to gaming conventions and on weekends.
What I basically don't do anymore as I've aged is ever dye my hair an unnatural color (at least not on purpose), never wear bright baby pink, never wear midriff-baring tops except for swimwear, never let my underwear show (this was harder to do when low-rise jeans were more popular), never wear flip-flops if the weather doesn't ACTUALLY permit it and never wear them to work or any formal occasion, never wear Crocs or Uggs or open-toed boots (or, really, any truly too-trendy footwear), never wear busy patterns, and I also never wear super skinny jeans because I find that even though I've gotten slender, I just don't think it's a good luck for someone out of their teens (other people can pull them off just fine, just not me).
Oh, and I never, ever, EVER show cleavage at work. That's less of an I-am-now-old thing and more because I work with a lot of males in their early twenties and before I made this rule, I often felt incredibly creeped out. I have a very smallish, modest, er, rack, but apparently twentysomething boys are easily distractable.
