In general, if you can't imagine roleplaying any character other than your current main, then chances are you need to mix things up. Doing so is going to require some grace and planning, but it's still times to call it lights out.
First, if you're a member of an organized guild or raid, talk to the person responsible for composition. It's kind of a raw move to simply disappear without warning. Even though it's your character and your fifteen bucks a month, you're still part of a team. Don't yoink the rug out from under your raid.
Tell your raid leader that you're no longer enjoying the character, and that it's time for you to move on. Be careful not to approach this as if you're asking for permission, but be open to counter-suggestions. If you're a tank or healer, for example, you might offer to continue filling that role as a different class. (If that's even viable for your raid.) The idea is to try and meet in the middle and be considerate of your friends and team mates. At the same time, if you're completely changing roles, hopefully there will still be a viable place for you in the future. It can be murky waters to navigate, but my best advice is to be open and honest.
Second, it's probably worth posting to your official forums or guild roleplay site that you're going to be changing characters soon. You'd be surprised how many roleplayers will feel hollow or cheated if they don't get to finish out story lines and tie up loose ends. By making a public announcement, you're giving everyone with whom you interact an opportunity to close out the last of those stories.
Third, it's time to plot your character's end. This is probably the most difficult part, because it's easy and tempting to turn the character finale into some huge, melodramatic event. It probably isn't realistic, for example, to have the Lich King personally show up and snuff out a random adventurer in Goldshire. Yet I've seen it happen twice in roleplay.
Make the decision whether this is a permanent retirement. If there is any chance at all that you might want to play the character again, do not end your character by means of a death scene. I know, it's tempting. It would be awesome to see who shows up at the funeral, who cries, who gives the best eulogy, and who wears the most mournful outfit. But if there's any chance you might want to play the character again, just don't go the death route. Coming back from the dead is so 1990s X-Men that it's just kind of lame nowadays.
And do not, under any circumstances, do the whole "my character faked his death" story. I promise, it's been done to death. It's probably been done better, but even if it was better, it was still hackneyed and overused then. Sometimes, I'm pretty sure no one ever dies in roleplay; they just fake their own death so convincingly that even the player thinks his character is dead. Be original: don't fake your own death.
So, if you think you might want to play the character again, and you're not going to fake your own death, how do you write yourself out? The options are actually fairly wide open. Maybe you're one of your faction's military, and you've been reassigned. Maybe you bought a farm in the rural areas of Azeroth, and you're going off to raise turnips. Heck, maybe you've become involved in a whirlwind romance, and you're going to sail the seas with your new lover.
The options are pretty wide and varied, and I always love hearing stories about how players voluntarily ended their character.
The last tip I want to share is that if you don't want give up your sweet level 80 character in raid gear ... don't. You can even rewrite a character as simply getting a name change. Plenty of people still play their character in raids in a purely out of character basis. Because there's nothing innately tying your character's roleplay to an in-game character, ending a character in roleplay doesn't mean deleting it off your character list.
All the World's a Stage is your source for roleplaying ideas, innovations and ironies. You might wonder what it's like to sacrifice spells for the story, or to totally immerse yourself in your roleplaying, or even how to RP on a non-RP server!