1. You can take a genre convention and deliberately screw it up, turn it on its head, turn it inside out, find out what makes it "serious" and not take it seriously at all by exaggerating some or all of it.
      My particular gimmick (if you will; and if you won't, I will, and have) is to base all the Kent Montana adventures, whether in novels or shorter fiction, on B-movies--all the cliches, all the minor characters, all the plots, and all the amazingly impossible coincidences within those plots. From there, I just poke fun at them through this exaggeration and just plain silliness. Goodnatured fun, I hasten to add; otherwise, the result would be simply mean-spirited, and not funny at all.
      For example, in The Mark of the Moderately Vicious Vampire (by my own thoroughly immodest self), the vampire is only moderately vicious because he has only one fang, the result of slamming into a church steeple early on in the story on account of he was seasick from coming across the stormy sea on his way to Maine. All the usual classic horror elements are there--the fog, the old house on the cliff, bats, rats, and the obligatory three sexy wives ... except that the tone very clearly states: You take this stuff seriously only at the peril of your sanity.
      In his Ebeneezum the Wizard series, Craig Shaw Gardner gives poor Eb what amounts to an allergy to magic. Every time he tries a spell or something, he sneezes. Frequently, the magic doesn't work; but when it does, the result is often chaotic. Also, Gardner's dragon sings and tap dances, his giant ferrets "eep" instead of roar, and his brownies and elves and dwarves, et al, really can't stand each other. It makes for interesting, and very funny, adventures when they have to team up to get at the bad guy(s).

2. Next is dialogue. People cracking jokes all the time does not a humorous story make. It's what the characters say in the current situation that carries the momentum and creates the humor. They are speaking what, for them, is normal; the situation makes what they say funny. I long ago discovered that using only slightly modified song lyrics can make for some interesting, if not downright dingbat, conversations.
      "You're so young," says the vampire to the woman he wants to be his wife.
      "But you're so old," says the woman.
      "Yes, this, Diana, I've been told," answers the vampire.
      If the reader gets it, fine; if he doesn't, I don't worry about it because there's something else waiting right around the corner. Equally as bad.
      Puns, too, are good in dialogue, but only if they're spoken straight; i.e., the characters aren't using them as puns, but as real speech.
      A man in that same novel sits on the beach throwing rocks at the birds running along the sand. In response to a question, he answers quite seriously, "As long as I live, I shall leave no tern unstoned."
      That's not a pun to him; it is to you.

3. Absurdity, when played straight, is a third technique for written humor. And that's pretty hard to do, when you think about it. The absurd situation must be written as if it is perfectly normal for the characters who have to live through it. If you, the writer, wink at the reader and let him know you're only joking, most of the humor, if not all of it, evaporates, and what you have left is stupid and unfunny. Which isn't to knock stupid, by the way. Sometimes stupid is smart. It's up to you to figure out when.
      The Reasonably Invisible Man deals with an angry British country/western singer and amateur scientist (absurd enough in it's own right, I guess). He's reasonably invisible because there are always one or two parts of him that do not vanish with the rest of him, no matter how hard he tries. And if you're thinking what I'm thinking, you're probably right, and it's disgusting and you should be ashamed of yourselves.

4. The types of characters themselves are vital. This may sound obvious, but it isn't, not really. Some writers, like Mr. Gardner in the Cineverse series, use primarily really silly characters in what passes for normal fantasy situations; others, like Esther Friesner, use normal characters mixed with semi-silly characters (usually the fantasy ones) to underscore the subsequent contrast, which is ... funny. And, of course, there are your basic normal characters in basically normal situations. Not funny at all until the situation gets a little skewed, the character tries to behave normally, and the result is chaos. Not necessarily funny for the character, but damn funny to someone as perverse as I am.