If it does not affect the final picture , leave it alone, thats all that matters, so clean the sensor, leave the rest alone, it only in your mind.
If it really bothers you, and you insist that you must clean your mirror, then use the following procedure:
remove the battery, remove the cards, remove the lens[of course] load the dishwasher with double amount the recommend of DAWN dishwashing liquid.
Place the camera wih the mirrow facing down, this is important to allow the water to not collect in the body, let the machine run through its cycle with double rinse.
Use this time to go browsing on line for a good deal on a new D3s, D700 or D7000.
Works all the time, you end up with a new camera with no dust, smudges or dirt.
Get a lens that's full of fungus and go diving with your dishwasher camera, it will amaze your friends.
TIP : always change you lenses only when you have to and in a dust free enviroment.