I start with a comprehensive list of all the recent songs that have been big hits - and then I go down that list and see if I can come up with funny ideas for them. I can always come up with ideas, but not necessarily good ones!
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'd like to be able to be more topical and timely and more of-the-moment and I think the way to do that is, instead of waiting until I have twelve songs to release all at once, just to release them as I come up with them.
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.'"
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.