I usually get very inspired by music, not just the lyrics (they also play an important role tho) but how the music itself makes me feel, maybe it reminds me of a character or maybe it makes me think of a situation. Then I proceed to listen that song in a bucle to sustain the inspiration.
Sometimes I just read o see something that reminds me of a character and the idea pops in my head.
In both cases I start to skecth the idea and if I like it then I proceed to polish it more, until I get the full color + rendered drawing!