Read a fabulous book recently- writing the romantic comedy, by Billy Mernit. He breaks down the old romance movies and goes into what made them work. It was worth reading just for the trip down memory lane! But I also found it useful as a writer. I think it's hard to write comedy, people either think you're funny or they don't, but you can learn the structures that work.
For me it all comes down to character. I ask myself, what does this character want and what can I do to stop her getting it? And then the fun begins.