Honestly there's no great secret to it. It comes down to a lot of browsing through artists' galleries and building up the biggest reference folder I can. I'm one of those people who enjoys categorizing and filing everything correctly based on fashion style, body shape, themes etc haha

As for worldbuilding, don't feel like you need to flesh everything out and write an elaborate history of everything that's happened and why everything's the way it is. Write out some ground rules for the tone and themes that you want it to hit and what can and can't exist without breaking the setting. I keep a google doc with everything I've added so far with details on the tech, climate, locations and infrastructure, and any time I have a new idea I can check it against what I've got so far to see if it fits!