A couple years ago Scott Adams laid out the keys to writing funny. It's excellent advice. A few up-front points about humor:

• A company's or an executive's ability to deploy humor is an undervalued asset in the business world.
• It is rare to find someone who is very funny and not smart.
• My two main filters on whether I want to spend time with someone: interestingness and sense of humor.
• Writing funny is harder than in-person humor. I discussed this a bit in my post The Best Jokes Are Hardest to Recall.

So, read Scott's advice on writing funny quoted below:

Picking a Topic
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The topic does half of your work. I look for topics that have at least one of the essential elements of humor:

Clever
Cute
Bizarre
Cruel
Naughty
Recognizable

In order for something to be funny, it has to have at least two of the six elements of humor….

Simple Sentences
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Keep your writing simple, as if you were sending a witty e-mail to a friend. Be smart, but not academic. Prune words that don’t make a difference.

Write About People
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It’s impossible to find humor in inanimate things. If you must write about an object or a concept, focus on how someone (usually you) thinks or feels or experiences those things. Humor is about people, period.

Write Visually
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Paint a funny picture with your words, but leave out any details that don’t serve the humor…

Leave Room for Imagination
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…Leaving out details allows readers to fill them in with whatever image strikes them as funniest. In effect, you let readers direct their own funny movie.

Funny Words
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Use “funny” words when you can. Here are some I used:

Mongolian
Herdsman
Vagina
Trouser
Shish Kabob
Storm drain
Johnson
Slap
Canoe

You can read that list of funny words totally out of context and it almost makes you laugh. Funny words are the ones that are familiar yet rarely used in conversation. It’s a bonus when those words have funny sounds to them, as do most of the ones in my list.

Pop Culture References
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References to popular culture often add humor. It’s funny that the world’s tallest man is retrieving a lost iPod, and not something generic such as a wallet. And it’s funny that his manhood is compared to Ryan Seacrest as opposed to something generic, such as an oak tree. Someone could write a thesis on why pop culture references are funny, but just accept it.

Animal analogies
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Animal references are funny. If you can’t think of anything funny, make some sort of animal/creature analogy. It’s easy, and it almost always works. I made these creature analogies in my post…

King salmon
Python

Exaggerate, then Exaggerate Some More
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Figure out what’s the worst that could happen with your topic, then multiple it by ten or more. Don’t say a mole is as big as a grapefruit. Say that mole is opening its own Starbucks. (Notice the pop culture reference of Starbucks.) The bigger the exaggeration, the funnier it is.

Near Logic
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Humor is about creating logic that a-a-a-lmost makes sense but doesn’t. No one in the real world could put gum on his penis and retrieve an iPod from a storm drain. But your brain allows you to imagine that working, while simultaneously knowing it can’t. That incongruity launches the laugh reflex.