Most of the time, dressing tzniusly just looks like a woman who works in an office. Unless you want to go with the seminary look. (The seminary look for fall and spring adds a hoodie, preferably from Stern.)
But there is one look that just screams frum girl: when you're obviously wearing a shell.
A shell is a plain, usually fitted, shirt worn under other shirts to allow them to conform to the rules of tznius. Shells are usually obvious as shells. They come in many colors and sleeve lengths, and they are underutilized as "real" shirts. (If you want to wear a shell as your outermost shirt, make sure to layer a tank top or something underneath it because shells are often thin and somewhat see-though. They also like to hug every "curve" in an unflattering way, sometimes even when worn with an undershirt.) Shells are usually used in neutral colors, such as white, black, gray, and off-white.
I suggest owning several black and white-ish shells because you will use them the most. I prefer 3/4 length shells because I hate having long sleeves feel "in the way." Similarly, I like my shells to be very fitted so that they don't bunch under the shirt or feel bulky. I have found shells, especially long-sleeve shells, at Nordstrom Rack pretty frequently on the clearance rack for under $10 each. However, I prefer Kosher Casual shells because they're the right fit for what I want.
This might be the frummiest outfit I own. Black pencil skirt from Nordstrom Rack, off-white shell from Kosher Casual, and blue untznius shirt from Nordstrom Rack. I thought my shirt was pretty secular-fashionable until I passed a mom wearing it in Brooklyn. While I was wearing mine too, of course. Awk-ward.
Now that I live in New York City (but still feel like a country mouse), I like playing "Jew or Not Jew" when I see women walking towards me in skirts. It's the subtle hints: hats with "too much" hair under them for secular standards, tichels of course, hair that might be a sheitel, and particular kinds of clothing. An outfit like the above (or any other secularly-"normal" shirt with an obvious shell) is almost always someone who appears up close to be an orthodox Jewish woman. Personally, I know it's an easy (lazy?) go-to outfit. Neutral skirt, neutral shell, add whatever shirt you grab. Easy peasy, and still very presentable.