CBS’ terrific comedy “How I Met Your Mother” is pulling out all the stops in its effort to get renewed. HIMYM made a nice splash by signing Alicia Silverstone, our favorite naked vegetarian, to appear in a multi-episode arc as Ted’s dermatologist/lover (and possible future wife, the “mother.”) Then the show created a tidal wave by snaring Britney Spears to guest star as an assistant to the dermatologist. But that was apparently too much for Silverstone, who dropped out rather than shrink under Brit’s shadow, according to media reports. So now Sarah Clarke (“Scrubs”) will play the dermatologist, but only for one ep (doesn’t appear she’ll be “the mother” after all). But all is not lost for Alicia. The show hopes to create a new character for her — after Britney’s gone.