Some outfits are obviously not work appropriate, but the line between what's okay for the office and what should be held aside for the weekend can be pretty fuzzy depending on your job and your age. If you work in an office with a traditional dress code, you probably can't wear a flirty sundress with boots and a cardigan no matter how young you are. However, in a more casual, laid back office, you might be able to get away with that any day of the week, right up until you retire.
When I graduated from college, I worked in a really casual office for a few years, and believe me, it was confusing as all get out. Being in my early-to-mid twenties and in shape, lots of the short, cute dresses shown in store windows fit me really well. Since that's what was in style, I tended to assume that was the equivilant of "ok for the office."
Coming off of the "Ally McBeal" era, short skirts in business-like prints were everywhere. Paired with a nice blouse and heels, I felt like I was the epitome of young office fashion ... until a coworker snarkily pointed out that, unless I was sitting juuuust so, she could totally see my undies. Whoops. And looking back, I can think of an embarrassing number of outfits that were just as bad, and it's not like I'm some Lindsay Lohan type of flasher (clearly, since it was undies that she saw and not, you know, something else). I just relied more on the mall displays and what I saw on television than I did on the good sense my mother gave me.
Does that mean you can't wear clothes that show your shape or a little bit of skin to work? No, I don't think so. But I think you need to be careful about it -- at any age -- and when in doubt, err on the side of caution.
As Liz suggests, it's helpful to identify the office uniform. This helps you determine whether to invest in skirts, slacks and pumps or something more casual. But what if your boss wears track pants one day and a suit and heels the next (as was the case in my old office)? You need to have your own set of hard and fast rules that show your sense of style, but keep you feeling professional and put together.
If I had it to do over, here are the three rules I would take death by stiletto to the eye before breaking:
Classy comes first. I hate thinking of skirt lengths or necklines as conservative, because I would never want someone to describe my style as such. Classy, however, is an adjective I'll take any day. Classy skirts and dresses don't show a bunch of thigh when you sit (and they certainly don't show your panties!). A blouse can show a bit of skin and still look classy -- if sexy is the first word that comes to mind, though, consider tossing on a camisole beneath it.
Save yoga pants for, well, yoga. Don't get me wrong -- I wear my yoga pants all the time at home, and often pair them with a fitted track jacket to run errands, but they have no place in an office. As Susan points out, yoga pants be part of an athletic look. Unless your office is in a gym, that's probably not appropriate.
Underwear stays under there. This isn't even an office rule -- this is an all the time rule. You guys, I do not want to see your bra strap. I definitely don't want to see your undies poking out of the back of your low-rise pants. I don't care if the bra strap is clear or matches your top; save it for your sweetie.
That being said, these rules might not work for your office or situation. Don't be shy -- let me know if you think I'm way off base, or if your office has far different requirements. And you could always follow Poppy's advice and break a few rules!