Two tips that come to mind in helping to build your skin is 1. Stay Humble, and 2. Develop a sense of humor.
The reason why I say to Stay Humble, contrary to another person's post about setting yourself up as better than others is because it's that precise mentality that breeds so many of the Rebecca Blacks and other douche nozzles that infest the Internet. So many people believe themselves to be great, awesome, better than anyone else, that they proceed to proclaim their superiority over the Internet, only to be flamed and verbally beaten into a bloody pulp. We need fewer of these, not more.
Now, I'm not saying you have to go around saying you're the worst person around. We all have our strengths, and I'm sure you're better than a LOT of people on the Internet in a LOT of things. But you're NOT the best, you're certainly not the best at everything, and most importantly, you have your flaws, as do we all. The trick is to admit and accept those flaws, and instead of hiding them, make sure they are always exposed. For instance, I am overweight, have been losing my hair since I hit 20, watch Anime more than I do American Television, and have played Video Games all my life. There's more to the list, but I'll stop there. By admitting these faults, as some would define them, and accepting them, they effectively become worthless as weapons against me, as I don't care if others know these faults, and will actually admit to them well before someone tries to "unearth" them to use against me.
Now, the second part to this, and is quite important if you're going to expose your faults like that, is, again, to develop a sense of humor. For best results, watch some stand-up, and notice how often comedians will pick at themselves, expose their own faults, and how often they'll play it up for laughs. By doing this, many of them become all the more endearing for admitting their own problems, and that they will probably never be able to fix them, or maybe that they never WANT to fix them. Furthermore, if someone ever attempts to use these faults as insult fodder, most comedians can sling back double whatever they're given, and do it with a level of class that the initial instigator can't compete against. If you can master this art, then none can possibly hurt you without taking greater damage in the process.
Oh, and one more thing, don't be afraid to take the moral highground and pass on some discussions. If a person is posting slanderous comments, examine the comments. The easiest target to go with is spelling/grammar. A person who doesn't know how to spell "the" correctly, much less half of their sentence, isn't exactly someone you should take seriously. After that, take some time to really examine the points they make. Are they even correct? Or are they just spouting friend-of-a-friend-level nonsense, constantly saying "I heard this" instead of "I read this". The more retarded the person becomes, the more ludicrous it becomes to even consider debating them. Of course, sometimes the debate may be necessary, but don't let yourself sink too far in. Remember, don't feed the Trolls, they start to breed after about the 50th feeding.
