According to this interview it appears that Stanley Tucci was inspired in his rendition of Caesar Flickerman by Graham Norton [Interview][1] Otherwise I have yet to find a quote from Suzanne Collins on her inspiration, but the search is still underway.

http://www.youtube.com/watch?feature=player_embedded&v=iILifPI97RI

  [1]: http://www.hggirlonfire.com/2015/04/13/video-stanley-tucci-says-he-based-caesar-flickerman-on-graham-norton/

From the video interview, 

> "When you create a character, you steal and cull from all sources. He's a cross between Wayne Newton and Jiminy Glick and there's a little piece of you Graham - the intelligence!"