Comment by colinjoy

Before you get bombarded with advice on how to make applications pretty, here goes my recommendation for how to get better at designing them:

Read the classics:

  - "The Inmates are running the Asylum" (A. Cooper)
  - "The Design of Everyday Things" (D. Norman) 
  - "Don't Make Me Think" (D. Krug)

Learn how to uncover problems and validate ideas (more books incoming!):

  - "Build Better Products" (L. Klein)
  - "Validating Product Ideas" (T. Sharon)

But above everything else: Build, test (with real users), improve, repeat ... nothing is more educating than watching your users fail to utilize the awesome design that you spent days on perfecting.