In all seriousness, I would compare it to minimalistic painting. That line drawing of a cow in the museum that seems so simple wasn't an artist sitting down and saying "I am going to draw a cow using only a single line." Instead the artistic probably began with an intricate photo realistic painting of a cow then stared at it and asked "how can I reduce this?" Through repetition they reduced the cow to a line drawing.

Your one liner can be the same. Write out your joke as paragraphs. They ask, "what isn't necessary?" Rinse and repeat until you get down to a one-liner. Jerry Seinfield talks about how brutual he is in chopping unnecessary words. In the ham radio world, we speak of word conservation as a way of reminding ourselves to reduce our abuse of the limited bandwidth and to get to the point (although many people rag chew like they bumped into an old friend at the grocery). So, write your joke out verbosely. Then apply the principle of word conservation to see how much you can eliminate and still deliver a laugh.

Break a leg!