Create your own professional uniform. For me it's a blazer, a cami and trousers, or a cardigan, a cami, a belt, and a skirt or trousers. That's pretty much it. I'll occasionally wear a knee length dress with heels but it's not part of my normal uniform.
Buy a few well-fitting jackets (blazers). They are my fashion must-haves. They dress up a t-shirt (or vest/cami) and jeans in an instant and make it look like you've made an effort. Even the most schleppy jeans are improved by a blazer. You need to try them on to make sure they fit right. E.g. the ones at Lane Bryant don't work for me (though they may for you), the Torrid ones are pretty much spot on all the time, I find.
If you're not into heels, there are all sorts of nice flats (I got a few pointy-toes cute ones at Sears online last year for around $10 each and I've worn them pretty regularly and they're still lovely). I go for pointy-toes because they lengthen the leg a bit and I think again, they make you look more like you've made an effort. But if not, regular round toe (ballet flats are also ok).
Another alternative is a cardigan (button-down sweater). Better still, get a wide elastic-backed belt (Torrid has an excellent selection) and belt it around the waist. I used to think this would emphasize the fat around my middle but it actually tucks in the fattest rolls and gives you (the illusion of) a much smaller-looking waist. And also it dresses up a vest/cami and trousers or a skirt. You can also just wear a cardigan by itself, but the belt really makes a difference.
There are lots of options for trousers. Dark blue denim (trouser-style) can be professional-looking, black pants of course, grey pants, pinstripes, and also muted traditional suiting patterns (dark plaid, houndstooth, herringbone).
I'm camera-shy and don't have many pics to show as examples, but I've found a couple from when I was getting a friend to help me pick out something last year. They're not technically work outfits since it was more for a brunch with friends, but hopefully you can see an e.g. of what I'm saying with the belt, cardigan and jackets. Sorry about the grotty mirror and kitchen situation going on behind me.