Unless PJs have dedicated camera bodies for each lens (not suggested), they'll get crud inside their camera. Each time a lens is changed, there's an opportunity for dust or a shard of metal to enter the camera body.
Additionally, since digital camera sensors are electronically charged, they're also dust and debris magnets.
The following isn't suggested by camera manufacturers, but it works for me. Folks with a camera under warranty or several camera bodies and plenty of time might not consider these options. Send the body to the manufacturer for cleaning
Check the sensor
Check the sensor before and after each cleaning attempt. The checks record progress or new problems.
Set the lens aperture to f/22 and take a properly exposed, out-of-focus shot of the sky or a broad-angle light. Check the resulting image for spots. Use the zoom or "func" button to get a close look at the image. Otherwise, download the image to a computer for closer inspection of the image.
If many spots are present (large spots or more than 15 small spots), consider cleaning the sensor.
Clean the lens mount
Gravity isn't just a good idea, it's the law. Consequently, lens mounts are designed to trap a certain amount of camera funk. If the lens mount is clean, the amount of recurring dust on the sensor can be greatly reduced.
In a clean, windless, well-lighted room, dip a cotton swab in alcohol and allow it to dry momentarily. Spin the moist cotton swab between a clean finger and thumb to tighten the cotton strands.
Remove the lens and gently run the tightened swab around the camera body's lens mount interior. Avoid catching it on mount edges, which could deposit cotton strands in the body and eventually on the sensor (a bigger problem than dust).
Place the lens back on the camera or move to the next step.
Clean the sensor
Buy a can of "canned air" from an office supply company. Make sure the can has an extension straw taped onto the side. If you started in the old days, this is what we used to blow dust off negatives (after the useless bulb blowers).
Remove the lens. Clean the ground glass plate (above the mirror) by holding the camera opening slightly downward and gently blowing off the plate. Then, carefully blow the dust off the mirror. If the can becomes cold, stop until it returns to room temperature.
Next, set the camera to manual at 30 seconds. Depress the shutter button to move the mirror into the up position. Very carefully blow off the sensor or internal filter.
NEVER get the blow straw near the sensor. It can scratch the sensor and cost huge bucks. Never hold the can upside down or do silly things to make it blow too cold. Keep the air stream moving evenly over the surface from left to right so the sensor isn't knocked out of alignment.
Turn the camera off to make the mirror return to its proper position. Place the lens back on the camera.
Check the sensor again
If many spots remain, repeat the steps for cleaning the sensor (the mirror and ground glass are only inconveniences for the PJ and don't affect the final images). If a few tiny spots remain, it's probably best to live with the tiny spots rather than risk a very expensive problem.
Most dust can be removed this way. If the spots won't blow off easily (mud or imbedded shards of metal), then it must be cleaned by the manufacturer.
Avoid future dust
Although dust is eventually going to get into the camera body, there are some helpful habits to minimize dust getting on the sensor.
* Never store or move a camera body without a body cap or lens.
* Keep the interior of camera bags, photo vests, etc. relatively clean to avoid getting dust and dirt on lenses while they're not in use.
* Shield the lens mount opening while changing lenses.
* Run a chamois cloth over the mounting end of a lens before mounting it on the camera. Obviously, do this while another lens is on the camera body.
* Avoid laying the camera on its back - particularly during storage periods. Again, gravity works.
* In hostile climate environments, only change lenses when absolutely necessary. Also consider changing lenses in a changing bag (or inside a folded jacket or towel). This minimizes desert dust, mud and water from getting in the opening.
* Treat environments with fine, powdery dust like a rain environment. Seal cameras in plastic sheeting or other rain covers to minimize the amount of dust getting at the camera.
* Use body caps when storing the camera (dead lenses work fine if there's not a cap around). If you don't know what a dead lens is, you're a lucky, lucky person. ;-}
Enough for now,