It's not easy being bored. It's even harder to bore other people. So often, there are witty, intelligent, or insightful things you can say, like, "This is a hell of a run-on sentence," or, "No it's not, you're just using a lot of comma splices."
I find the best way to bore people is to tell them things they never wanted to know. For example: if you eat something with gravy and cheese for dinner & don't wash your mustache out afterwards, the next morning you're going to wake up with a nauseating odor stuck in your nostrils.
Another good way to bore people is to talk about work. "I put some pipe up today. It was one inch EMT. I used a bender to make offsets. Did you know that the multiplier of the distance between bends in an offset is the cosecant of the desired angle's sine?" If you want to go for bonus points, you can also be arrogant: "I'm much smarter than Blogger, because the word cosecant isn't in its spell-check dictionary!"
However, I've found the best way to be boring is to start conversations about things no one else cares about, such as, "Peter Cushing really doesn't count as a Doctor Who, because he's just a human and his movies were re-makes of Dalek TV episodes anyway," or, "Do you know I saw a ten-foot walrus masturbate at the Brooklyn Aquarium using all four flippers and his tail?"
Please feel free to add your own.