I've been reading Comedy Rules, the entertaining memoirs of Jonathan Lynn offered as 150 tips for scriptwriters. As his successful writing career culminated in Yes Prime Minister ~ an updated theatrical version of which is touring nationally as I write ~ his witty anecdotes also embody substantial useful advice. There's discussion on the role of comedy in performance (self-recognition and anger management, to précis), the relationship of farce to tragedy (two sides of the same coin ~ action driven by hideous dilemmas), the importance of dramatic irony, the need for 'treacle cutting' with a dry comment after moist emotion, the reason suspense works better than surprise, the causal connection between character and plot, and a great deal more too. One of my favourite rules is number 71, relevant to all creative activity and not just writing: Where do ideas come from? Some I get from newspapers. I also get them just from gazing out of the window.... sometimes when I’m out walking, or when driving. But where do they come from? I think that they come from programming your brain in advance, by setting it a problem and then leaving your subconscious to mull it over without supervision. 
I cherish also number 124: Polish the dialogue until it sparkles. Great dialogue, though it may seem realistic, seldom is. It requires artistry, impeccable phrasing, the perfect vocabulary for the character, and the script should sustain a level of wit (whether or not the characters are aware of it) that is not possible in real life.  
And for a light look at how to to get 'em laughing, The Last Supper sketch is remembered fondly by the author from his days with the Monty Python crew.