No matter how well you store your Nikon D90 or practice the invert-change-lens rule, or for that matter, don't change lens at all, you are bound to have dust settled in the mirror section and sensor section of the camera.
I was doing a routine inspection by simply looking through the lens towards a light source and voila! Dust speckles are deposited everywhere on the mirror section! There's even smudges on the lower section of the mirror. Now, I have not activated the "Lock mirror up for cleaning" function in the Setup Menu, but I decide to give the sensor the benefit of a doubt that it is still clean. Besides, sensor cleaning should be done by the local Nikon technicians if you clearly lack a steady hands and right tools.
With a lint-free dry cloth (usually come standard with purchase of a good LCD monitor or laptop), I gently removed the lens and began cleaning the smudges and dust particles off the mirror section. Shortly after, the job is done. Talking about sensor cleaning, its a good practice to enable it when you off/on the camera. This way, you leave no occasion to chance that dust deposits starts accumulating on the sensor grid.
Note, for zoom lens, air is drawn into the lens when you zoom in and out, giving occasion for dust to get into your body with the lens tightly attached!