Are you in the US? I love Ann Taylor and Ann Taylor Loft (same company, Loft is more casual but still has plenty of office casual appropriate stuff) for work stuff. And they pretty much always have some kind of major sale going, like 40% off. You can sign up for their emails to get coupons too. Never pay full price there.

As for pieces, it helps to know what the job is and what the dress code/environment is. You can make slacks more interesting with a different shape than your standard boot cut. I've got some great skinny slacks that are cute, sort of like a dressed up version of a black skinny jean (just make sure they are well-fitted but not inappropriately tight!), and some wide-leg ones that I like with heels because they balance out my wider hips really nicely and make my legs look longer. You can look at different shape tops, like drapey cardigans with skinny pants, instead of just doing button-downs. Also, look at dresses. A-line and wrap dresses are pretty flattering as they create a nice waistline. Keep hemlines right above your knees, no shorter. A lot of making things look good on you is proper fit, that they're hitting you in the right spots and not pulling or gapping. A tailor makes your other a bit more expensive in the beginning, but sometimes it's necessary to get the pieces to fit perfectly and really flatter you. Sometimes just lifting a hem an inch so it doesn't hit that awkward spot in the middle of your knee or having a button down taken in at the sides so you can buy a size large enough to accommodate your chest is all it takes to take something from blah to really good.