I'd recommend an audio cd set (you can torrent it) called Vocal Power by Roger Love. I do some acting and public speaking stuff and it helped sort quite a few things out as far as vocal technique goes.

In my opinion though the thing that made Hitchens entertaining and persuasive in debates was his ability to structure an argument properly, which was due to him being an awesome rhetorical writer. For that I'd try some books on rhetoric and logic. As far as the humour side goes there are very few books on comedy writing and most are pretty worthless anyway.