Here are some guidelines I’ve developed to help ensure your work hits the reader’s funny bone:

• Set the right pace to build anticipation. Don’t rush to get to the punchline but don’t drag it out either.
• Make use of exaggerations and hyperboles. Some writers may differ on this but I try to keep my exaggerations still within the realm of possibility–just enough to stretch the reader’s imagination.
• Be sure to have some unexpected twists and turns to keep your readers on the edge of their seats. Surprise endings are key.
• Use very vivid language and sensory details (i.e., the cactus had as much allure as a Brillo pad).
• Make every word count. If it’s not funny or part of the setup–delete it.
• Don’t use any framing language. SHOW, DON’T TELL!
• Write snappy, witty external dialogue and internal monologues. 
• Read it out loud or better yet have a friend read it out loud to you and see if it’s as funny as you thought it was. Adjust accordingly. 
• Share your draft with friends and family and ask them to be very candid about what’s funny and not funny. Keep what is, and delete or massage what’s not. No room for weeds.
• If you submit to a contest, see if there’s an opportunity to receive feedback from the judges. Will improve your work tremendously.
• Good luck!