How do you set boundaries with friends who think the world revolves around them?

so my friend and I work in the same place and lately she has been driving me crazy. I mean, she relies on me for almost everything that she needs. If she is hungry she wants me to bring her food, she aks for other favors as well. Its gotten to the point that I am fed up with it. She acts like my number one goal is to serve her and her needs and when I tell her I am working she gets all upset. How do I fix the problem I have created without hurting her much or losing the friendship?