The majority of the dust is vacuumed in through the lenses. As the lens is zoomed in and out air travels through the lens carrying dust particles and deposits them in the mirror box/shutter area. When the shutter is fired the CCD is exposed and dust will gravitate to the CCD.
To avoid dust from entering your camera: use a blower brush on your lenses to remove some of the surface dust.
You can use the canned air on the lenses but DO NOT use it on the sensor/filter.
Changing lenses will also allow dust to enter the mirror box area: When changing the lenses face the camera downwards.
Even after a cleaning unavoidably dust can return to the CCD/Filter surface.

Dust is very intrusive. It can be vacuumed in through a lens, occur when you are changing lenses and the environment you are shooting in. Have a look at a glass top table each day and see how much dust gathers on the tables surface. This is what is in the atmosphere while you are shooting.