My main recommendation would be to never start with a blank canvas. I usually start by picking a random photo (or one of my other paintings) and completely mess with it in photoshop, just to get a random bunch of textures and values, and when you feel like it start painting what you "see" in these shapes. It helps inspirations and it also gives that cool textured look.
Basically, don't open photoshop while thinking "I'm gonna paint a big sci-fi city", just mess around without really thinking about what you're doing, almost instinctly.
Hope it helps