Think after before
In the internet it can be "easier" to write mean comments since you never have to face the receiver face to face. That doesn't mean its more ok to do so - to get a mean comment hurts as much, whether you get it in your cell phone, in Facebook, in the schools hallway or in the lunch room. Think about how the person could respond to your message before you leave a comment.
Be nice
Decide how you want to be towards others and what kind of mood you want to contribute with in the internet. Also decide what you don't want to do, like writing mean comments or like a photo taken behind someones back. Instead, fill the comments field with positive comments, which not only makes the person happy, but you´re also contributing to a overall positive mood.
Start a "being a good friend-blog"
Many blogs has a negative content. Do the opposite - start a blog that spreads positive thoughts and stories through others.
Disregard fradulent (non-serious) sites
As an user of a site you affect the content. Use your users power by disregarding sites with offensive conent, bad user possibilities or moderators that doesn't do anything against mean posts.
Report offenses
If you get exposed or you see someone else being it, report the offense. In serious sites there are moderators who reads, decides and obstructs the user who is doing the offense. If you go to school you´re protected by the schools law, where it states that the school needs to take action against bullying that happens in the internet, if it affects the school day in a bad way.