Remember to be respectful of others when you are on the internet. Whether you are writing an email or chatting online or writing on someone's Facebook or My Space page, remember to be kind, as you would if you were speaking to them in person.
It can be easy to forget how your words can affect someone if you aren't face to face with them. Be kind to them; if you really need to discuss a difficult topic with someone, show them respect and do so in person. However, if you think you or maybe even a friend of yours is being bullied online, be sure to tell a parent or teacher.
Being polite on the internet can carry over into every other part of your life! Especially considering how much communication people do online and through texting and email these days. If you are polite on the internet, it will carry over into your face to face conversations, too! You will become a more polite person overall.
Make a commitment to write something positive and encouraging to one friend each day. You can write something on their Facebook or MySpace page, you can send them an email, or just offer an encouraging word while you're chatting on instant messenger. Each day pick someone that you are going to go out of your way to be nice to online.
If you struggle with encouraging others, that's okay! You can still be polite: respond to people when the email you or start a chat with you. Explain you don't have much time to chat, but acknowledge them and say hello. If being polite online is something you really struggle with, then simply remember the wise adage: "If you can't think of something nice to say, don't say anything at all."
Some great "netiquette" tips.
Polite tips for Facebook users.