Year-round wardrobe essentials?

I work in a clinic as a medical biller and coder, and most of my clothes are fairly casual. What are some inexpensive pieces that I just HAVE to have in my closet to dress up each outfit a little more? Suggest some pants, shoes, shirts, whatever you think necessary, and if you could, provide links. Also, I will be going back to school, so I need to be comfortable from 5 a.m to 10 p.m without wardrobe malfunctions. And I'm 23 :) Thank you!!!