How to prevent dust, dirt or moisture from getting on the image sensor (or mirror) inside the camera.

Since DSLR, SLT, and NEX cameras use interchangeable lenses, there is always the possibility that a piece of dust, spec of dirt or moisture may find its way inside your camera and ultimately onto the image sensor (or mirror) when changing lenses. There are precautions, however, to help avoid this situation.

Note: DSLR and SLT cameras have mirrors, but NEX cameras do not.

• Turn off the camera before changing the lens - image sensors can have an electric charge that attracts dust.
• Only change lenses in a clean environment, preferably indoors away from any fans, vents or windows.
• Check the lens before attaching it to the camera; if dust is found on the lens a blower can be used to remove it.
• Have the lens you want to attach ready beforehand so the lens opening of the camera is exposed for as little time as possible.
• Position the camera downward when removing and attaching a lens to prevent dust or dirt from falling into the lens opening.
• Attach a dust cap to the front of the camera after removing the lens if not using the camera for a prolonged amount of time.
• Attach a dust cap to the lens you removed from the camera.

As stated previously, there is still a chance of dust, dirt or moisture finding its way inside a camera with interchangeable lenses. Moisture may still accumulate if the air inside the camera is dry and the air in the outside environment is moist. Following the guidelines above, however, should help minimize these circumstances.

About the way of cleaning image sensor, refer to the following.
How to clean the lens and image sensor