It's usually by being a jerk.
Ever wondered how people get labeled as trolls, flamers, or just
plain wind up being hated by the forum communities? Well wonder no more!
Join Charabis as he takes a brief look at forum etiquette and how you can be a
valued member of any forum you're on!
...If you simply want to agree with someone by saying, "Me
too!" or something similar, you're better off not posting. Make sure you
actually have something to say or add to the conversation before posting...
These and other useful tidbits are to be found at Ten Ton Hammer's G&H site.