I find that a layered look on top helps to up the "business" part of "business casual." A blazer over a top. A cardigan over a top. A button down shirt over a tank top. Some vests will also work as that top layer.
Do a little Googling on "capsule wardrobe." The basic idea is to pick one main color, say black or navy, and then have a couple of accent colors, with everything working together.
So you might have a black jacket, skirt and pants that are made to go together. Then a gray or black cardigan, and a red one. Three button down shirts--white, a black and white stripe and a red-and-white print. Some knit tops--white, grey, black and red, with a print/stripe/check thrown in for interest. A pair of gray slacks. You can wear the jacket and both cardigans with all the tops, and with all the bottoms.
Then maybe a second capsule, based on navy or brown or khaki.
And then you can mix and match between the two. Red goes with navy, brown and khaki. Gray goes with navy. Black goes with khaki.
When you are shopping with a purpose, in this case to upgrade your wardrobe, as opposed to just shopping for fun, I think it helps to have a list of what you need: a white button down that looks good tucked in, a red top to go with the gray sweater and black pants, a print top to liven up the black suit.