In fact, when I come up with an idea for a parody I try to resist the urge to Google the idea to see if someone has done it already because the answer is almost always, "Yes, of course they have, they've thought of it!"
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.
I was a huge fan of 'Mad' magazine when I was 11, 12, 13 years old. I'd scour used bookstores trying to find back issues, and I'd wait at the newsstand for a new issue to come out. My life revolved around it.
Somebody will come up to me after a show and have me sign their arm, and the next time I see them my autograph has been permanently inscribed on their arm.
I was able to come up with a couple articles for the magazine, I was able to solicit help from a bunch of my friends to contribute pieces: Patton Oswalt, Seth Green, Emo Phillips, Chris Hardwick, John Hodgman, and more. It's very much a "Weird Al" themed issue, so I'd like to think that there's a lot of "Weird Al" flavor throughout but I think it'd be generous really to call me an editor.
It's hard to really articulate what the parameters are that make one song parody-able and another song not, but if I can come up with a good enough idea for it, I go for it, and if not, then I have to move on.
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.
I'm a music fan, and I can listen to the radio, or music, without thinking, "How am I going to screw this up?" [Laughs] If I'm really actively trying to think of a parody, then I'll have my antenna out, and be a little more proactive about it.
At this point I've got a bit of a track record. So people realize that when 'Weird Al' wants to go parody, it's not meant to make them look bad... it's meant to be a tribute.
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.