If you keep getting banned or kicked on servers then you should read this...

IDOLING:

Usually if you have a rude username like "boobs" of "motherfuckingretard" then you will be kicked for IDOLING.
After getting kicked you should NOT go back onto the server until you have changed your username.

SWEARING:

This usually gets you banned on servers like Microkraft so DO NOT SWEAR or don't bother going onto the server.

HACKING:

Do not hack. Never do it.
Hacking = INSTA-BAN!
This hacking section includes things like hacking the server, flyhacking, and lots of other things.

GRIEFING:

Griefing is a bad thing. Some servers do tolerate it, but other servers do not. Don't grief on creative servers because it could be a permanent ban for you!

RACISM:

Most servers will NOT tolerate that kind of thing.

PUTDOWNS:

Some servers do not care if you put others down, but don't do it. Because no one likes being put down.

ADMINS AND MODERATORS:

Do NOT piss the admins and moderators off. You could get insta-banned!!!

BUKKIT SERVERS:

(What TOBEwORwNOTTOBE said) Also be very careful in bukkit servers cause of that plugin that bans you from all bukkit servers that has it.

There are a couple of other ways to get banned from servers like:

Sexualism.

Killing others on creative servers. (Some creative servers might have PVP.)

Killing people without the server being PVP.