A few tricks you can play on yourself to break out of a rut:
Use skills you have never used, or a set of skills you have never used in combination before. If you play mages, make a warrior. Don't let your character use any of the mage skills your characters usually use. It can feel a bit like trying to write with your off-hand sometimes: awkward and klutzy and all wrong, but it can be useful in breaking habits and letting you experience things in new ways.
Choose a fictional character you know well and play as that character. Try to stay in character. Take only those actions that your fictional character would take. This works with real-life people too. If you know somebody in real life inside and out, take them as a model. Play as your best friend, one of your parents, a brother or sister, anybody. The more intimately you know this person the easier it will be to stay in character.
Use quest mods. Sometimes doing brand-new quests can kindle your interest and keep you playing a character.
Set your own goals. Accumulate one million Septims. Break into every interior in the game without getting caught. Collect all of one thing in the game: tankards, shoes, anything that strikes your fancy. One of my characters collected skulls. Finding all of the skulls in the game was a kind of miniature home-made "main quest" for that bizarre character.
Keep a journal. Writing down your character's activities can turn a game into a narrative. You may find you become more personally involved in the character's journey if you have a written record that you can refer to and which you add to on a regular basis. Sometimes, when we live in the moment, we lose sight of where we have been and where we are going and the experience can begin to seem meaningless and we lose interest.
Related to the above: post your character's adventures in the http://www.gamesas.com/topic/1595472-character-updates-18/ thread here. Take screenshots and post your screenshots in the http://www.gamesas.com/topic/1606364-oblivion-screenshot-thread-25/ Getting feedback on your character from other people may help maintain your interest in that character.
Finally, here are a couple of older threads with some neat ideas that might help you out:
http://www.gamesas.com/topic/1115805-completely-random-character/ by gpstr
http://www.gamesas.com/topic/1327345-pretend-mods-for-vanilla-games/ by Renee Gade III
Good luck!