Watch a sci-fi movie, maybe. Research real-life concepts, plausible creations, or crazy ideas. Things like that really get me going. For only a few seconds of some guy's thrown-together slideshow of images from Defying Gravity's pilot episode, a spaceship orbiting Mars was shown. I haven't been able to find it anywhere else (barring the actual show), but I was so inspired by that shot that I had to make it in KSP, which I did. It's currently flying a test mission around the Mun and Minmus, and the design will soon be sent all around the Kerbolar system.