Uh, "filler", would be what I call theme-driven concepts, which I usually come up with when I thinking about a subject that's pretty broad, like, for instance, "Why does everyone hate California?" or, "What's different about Christmas in California, as opposed to say, Wisconsin." When, suddenly, you find yourself writing a Christmas book, you see opportunities to plug them in.
My advice would be to not come up with filler to put in your stories, period. I can get away with a lot of shit that you guys, as new writers, cannot. Some of the pieces in Stupidest Angel are much more about the generic holiday season, than they are about the story at hand, and consequently, they stand out. Also, much of TSA depends on a setting and characters which I have already created, and therefore I was high-stepping to not say the same thing over and over again. For instance, in the previous books that featured Mavis, the bartender, there were extended pieces describing her gradually being replaced by spare parts, but it would have been overkill to go there again. Something like Gabe doing his experiments on the rats might be filler in another context, because it really doesn't have anything to do with the story, and it's only in there because I thought it would be funny. The bottom line, even in my own stuff, is that if it appears to be "filler", it probably shouldn't be there. Comedy gives you a lot of leeway, because as long as what you writing is funny, you'll be forgiven a little sloppiness.
Comedy tends to be a spontaneous artform, a reaction, if you will, to stimuli. The craft and timing may come later, but the initial reaction is usually what makes something funny. That being said, when something strikes me as funny, I tend to write it down. Later, if I can plug it into a book, I'll use it in context, if I can't, I'll save it up and just use it in a presentation when I'm touring, or I'll just hold on until it's too stale to use. (I have great fax machine material that will never be used. ) This is one of the reasons that I recommend a writer keeping a journal, and writing down all those ideas, because you will forget them later when you need them.