For ideas, just spur of the moment stuff. This extends into stuff like art for a project and further planning. From there, I look at what I want to do, what pulled off similar ideas for comparison, what I like most in an idea and why I like it. I'm not afraid to admit if I'm influenced by an outside source - that honest allows me to go all-in with a direction, so-long as I'm not going too far and outright ripping something off.
Those are just ideas, but to demonstrate with what I've been doing, I'm making a game that's an endless runner. I know the tone will usually be nighttime. I know that I want combo attacks, flight, and the ability to grab and throw enemies. While sketching the main character, I can recognize what I was exposed to in order to influence these ideas: Batman for a dark, urban setting, Klonoa for enemy manipulation, Kirby for fluid movement nuance, etc... Rather than delude myself into thinking that I can replicate what I loved about the source materials, I can use them as starting points to flesh out certain things into an original idea (or, rather, as original as anything else).