It depends on the series, really. Slayers had a lot of filler material, and it seemed to work well, but that's because the plot of those shows was secondary to the humor. On the other hand, I was largely unimpressed with Rurouni Kenshin's filler episodes. Then again, had they had more filler episodes so that the anime didn't run too close to the manga, they might have been able to animate the Earthly Vengeance story, and that would have been cool as hell.
I say nothing at all, unless the show is intentionally going on a different path than the manga, as is the case of Full Metal Alchemist, and absolutely NO CLIP SHOWS,cause I don't care if newbs start watching late in the game, they can watch reruns for all I care, and I love long run-on sentences, they are freakin awesome,and comas are my friends, and my greatest enemies...<_<.....>_>....,peanuts.