Ask HN: How can I develop my design skills (coming from sw engineering)?

One of the difficulties I get during the development of side projects is that I always get stuck with "programmer art" and not really able to create visually appealing representations of the concepts I have in my mind.

For example, I have several web app / mobile app ideas, I've created entire application models and actual prototypes, but get discouraged by the presentation part of it.

I figured that learning some design concepts and techniques might help, but whenever I work with actual designers I'm always impressed by the quality and speed at which they are able to create these visual concepts.

Is there any way I could learn (preferably from free resources) these concepts and techniques? The same trouble applies when creating presentations or proposals - the ideas are there but getting from idea to visuals is a pain.

Thanks in advance for your help and tips!