The early beginnings of my current comic date back almost 8 years, when I completed my PhD and suddently realized I had had no creative life at all for the whole time of my grad studies. I had no old project to continue, nothing ready to start working on, for the first time in my life. It felt very bad. I felt empty and useless.
So I challenged myself to start getting creative again.
The first thing I did was to make up a story for a Skyrim RP, that would absolutely require some modding (so I was forced to imagine things outside of what the game was already proposing, and actually get creative). Worked nice. The only thing that remains of this RP in my comic is the name of one of my characters and the fact he lives in marshlands.
Then when I was driving for substantial periods of time, I would challenge myself to make stories on the spot, entirely improvised and without thinking of where they were going (almost like automatic writing). I have over a hundred hours of these (I recorded everything) and they have been feeding my comic since then.
Because of the way I write, described above, the influences in my comic are multiple, and pretty much everything present in my cultural, personal, social life (or my past) can end up in my comic (in a generally pretty indirect way, though).