It was difficult to get into my friends' rock bands when I was a teenager. They somehow didn't see the need for an accordion player. That's when I realized that I had to find my own path in life.
There's a lot of different ways that a song would be a challenge to parody. There are a lot of songs that would ostensibly be a good candidate for parody, yet I can't think of a clever enough idea. Some songs are too repetitive for me to be able to fashion a humorous set of lyrics around. Some songs flat-out just don't work creatively for me.
I cut my teeth playing rock songs on the accordion when I was a teenager and my friends always thought that was extremely amusing. I think that was the genesis of my polka medleys, because every rock song I played on the accordion just sounded like a polka and my friends thought it was funny. So that was a joke that I continue up to this very day.
My own personal tastes don't really have an effect on whether song is a parody target or not. But having said that, I try to pick songs that I actually like because I realize that I have to live with these songs for a long time, from when I'm working on them in the studio to possibly playing them onstage for the rest of my life. So I try not to pick songs that I know would drive me crazy.
I have very mixed feelings about it. On one hand, I’m concerned that the rampant downloading of my copyright-protected material over the Internet is severely eating into my album sales and having a decidedly adverse effect on my career. On the other hand, I can get all the Metallica songs I want for FREE! WOW!
I write and write and write, and then I edit it down to the parts that I think are amusing, or that help the storyline, or I'll write a notebook full of ideas of anecdotes or story points, and then I'll try and arrange them in a way that they would tell a semi-cohesive story.