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.
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 tend to enunciate pretty well. It's always seemed that my voice is one of those voices that people can recognize pretty easily - which has been a bit of a drawback for some characters because you're supposed to lose yourself in the character, but sometimes people look at a character and go "Oh, it's 'Weird Al.'"
So that's why one of my rules of parody writing is that it's gotta be funny regardless of whether you know the source material. It has to work on its own merit.
My process for the parodies is that I get an idea for a song and then get approval from the artist and then go in and record it and probably try to get it out as soon as possible.