The more responsive design work I have done, the more important I have found it to get designs up and running in a browser environment as soon as possible. If you are a designer as well as a developer, that simplifies matters. As soon as you have enough of a feel, visually, for what you need, you can get it prototyped in a browser and develop the idea further in a browser environment. This approach can be embraced more fully by letting go of high-fidelity full-page mock-ups altogether. Instead, consider things like Style Tiles—positioned between a moodboard and full mockup. The introduction to Style Tiles (http://styletil.es/) describes them as:

I've found graphical deliverables of this nature can be useful for presenting and communicating look and feel between stakeholders without resorting to the endless rounds of composites.

.rule {
  /* Smallest viewport size styles */
}

@media (min-width: 40em) {
  .rule {
    /* Medium viewport size changes */
  }
}

@media (min-width: 70em) {
  .rule {
    /* Larger viewport size changes */
  }
}