Of course, sometimes the character goes just the opposite way, and no matter what you do, your stories fall flat and you don't feel enthusiastic about logging on. It may be because you can't stand the character's personality anymore, it may be because your normal group of friends has hit an RP rut or even taken a break for a while themselves, it may be a variety of other things. Recently, Anna of Too Many Annas wrote a pretty awesome article about how to refresh a flagging RP character using 5 Rs: Relax and Rest, Reintegrate, Refresh the Character, Retcon, and Reroll/Reskin.
What's sort of cool about this list is that you can even apply a lot of the principles to characters in general, even if you're a non-RPer. Anna's covered the RP angle pretty well, so I'll mostly focus on this other angle.
Relax and Rest
This is pretty straight forward. Sometimes you just need some time off to recoup. Maybe you need to play an alt for a while, or maybe you just need to unplug for a week or two, play some other games, take up a new hobby, and mix things up a bit so you can return to the game with a new perspective. I know whenever I get in a rut on WoW, sometimes not playing for a week or so does wonders to get me back on track.
Reintegrate and Refresh the Character
In roleplay terms, this is really integral for every day experience, I think. Don't wait for other people to start the roleplay, start it yourself. Don't be afraid to break into a new group of people, or try a new locale for your RP. Stay involved, stay outgoing, and stay different.
You can do the same thing for gameplay. If you're used to waiting for someone else to start that dungeon run, start it yourself. If you're always running dungeons with the same 4 people, pick 4 new people. No, I don't mean your original 4 people suck, but sometimes a different class composition for a group can shake things up and give you a chance to see your role from a new perspective. If you always do the same few dailies every day, try doing some dailies you almost never do. Even if you've given up Storm Peaks for the Tournament, you may find that a couple days of spear thrusting is just what you need.
Retcon and Reroll/Reskin
For a Roleplay character, sometimes you may just have to give up, admit you've roleplayed a character into a corner, and do some quick retconning to change a pivotal event or a character trait. Sometimes, you may go extreme pay the cash to change their name, looks, or even gender to get a new character out of the same template. Sometimes, you have to give it all up and roll a new class, and send your old character to bank alt land.
But even this can apply to gameplay considerations as well. If your same old class and spec is making you bored, visit the trainer and wipe your talents. Try another talent tree, or at least a slightly different build. You might be surprised at how much a couple of talent points can change your whole outlook or playstyle. This is, of course, a bit harder to do if you're a raider who is expected to fill certain roles by your raid group, but hey, there's always changing back just before raid time - or keeping your "real" spec on the one of your dual specs.
In short, though, this is a pretty neat list. If you're an RPer or interested in RP issues, it's worth a read, and if you aren't (or if you are since RPers delve into the other aspects of the game too), the basic principles still have some lessons to give from the gameplay side of things. Check it out.