I know the feeling, my humour has two settings: "Snark" and "Dead Babies". Neither of which are that good for lighthearted fun-poking.
Most of what I read being serious Sci-fi or Fantasty doesn't help too much either.

This is the reason there were four years between the time I joined and the time I wrote my first actual PPC story. {= p

I know what you mean about Discworld, too--I tend to mimic the style of things I've just read. Hence, there are times I can spot definite Pratchettian moments in my writing, and other spots are more Tolkien-esque. It's a little weird.

However, there are a few things I've noticed over the years, and maybe they'll help. The main thing is to get as specific as you can when describing things. Don't say "it was totally bizarre" when you can say "it was like a duck trying to tap-dance while bouncing on a trampoline." ... I have no idea what that description might apply to, but I'm pretty sure somebody somewhere will giggle when they read it, because it's ridiculous, and ridiculous is funny.

Another thing is timing, and that's hard to control in writing. You have to direct your readers' eye to see what you want when you want for maximum impact, like a comedian building up for the punchline. If you slip and reveal the punch during the setup, it's not funny anymore.

There is also exaggeration/hyperbole, either in the over-the-top sense or in vast understatement. Using these two things (with specific description) to create contrast can also work. We're pretty well wired to respond to contrast, so that's really a separate tip unto itself. That's why having "odd couple" agents works so well.

And that's all I can think of at the moment. For a study in the art of awesomely amusing PPCs, I recommend the works of Tungsten_Monk, now available here on the PPC Lost Tales archive.

~Neshomeh

I'm pretty humor impaired too, perhaps even more than you. It's something I've never really understood well...

Yeah.

You don't have to use giggle-humour to make others laugh - a well-placed dry or sarcastic comment could do the same. Also, if you (or your characters) take everything Utterly Seriously, it could work out to be amusing, too. Of course, it won't always, so I suppose that in itself is not much help.

...I think your remark about about Discworld is pretty humorous in itself.
As a formerly humour-impaired one myself, the best thing to do in my opinion is resort to sarcasm... It's not the best approach to make new friends, but it's exceptional for entertaining existing friends or, in cases like this one, to make readers laugh (or at least I laugh at sarcastic remarks).