If you're an active photographer, at one time or another, dirt or dust will get on or into your camera. This can happen on a camping trip, at a sports event, at family affairs - almost any outdoor and many indoor activities where dust could be present.
Having the camera in a gadget bag when not in use is the best way of keeping it clean, but this isn't always practical. So if the camera does inadvertently get dirty (hopefully, it will never happen advertently), there are ways of cleaning it that will prevent things from getting worse.
If dust settles on the outside of the camera, removing it is usually just a matter of blowing or brushing it off with a soft, dry cloth or brush. When eliminating dirt or dust from a camera, don't wipe it off. Use a flicking motion so that the stuff does not become a permanent design in the camera casing.
In 35mm and APS (Advanced Photo System) photography, dirt or dust can get inside the camera when you're changing film, or while changing lenses on an SLR. Sometimes on a dusty, windy day, dust will settle in crevices when you're using the camera, and sort of drop into the camera when the back is opened or the lens is removed. And a camera, any camera, just seems to absorb dust from the air when it hasn't been used for some time.
When dust or dirt gets inside the camera, don't try to blow it out. This can cause the offensive stuff to migrate further inside the camera where it can cause major problems. One way of removing the dirt is to pick it out with a good-quality fine-bristled artist's brush. Don't use a cheap brush to clean the camera because this can add dirt of it's own, and bristles have been known to break off inside a camera. Also, these brushes could possibly scratch delicate parts of the camera mechanism.
If you have an SLR, you might notice dirt when you look through the viewfinder. This is nothing to worry about. These spots are only within the viewing system so they have absolutely no effect on the film.
Spots of dirt on the reflex mirror are another matter. Individually they're not bad, but enough of them could affect the image quality, and of course, they can move to the interior of the camera. Since the mirror system is one of the most delicate parts of the camera, be extremely careful in removing dust. If the spots can't be removed by just barely touching them with a brush, take the camera to a lens specialist or photo shop.
Rather than brushing, picking or flicking bits of dust or dirt out of the camera, vacuuming is a much better way of removing them. There are several very small battery operated vacuum devices on the market just for this purpose.
Before you run out and buy a tiny vacuum cleaner, I have a better, and cheaper, idea. Find an old ball-point pen (Bic works best), and remove both ends. Then cut a two-inch circle from an old, well-washed man's handkerchief and place it over one end of the tube. Hold the cloth in place with a small rubber band or a 5/16 inch rubber O-ring. You now have a vacuum cleaner.
To use this device, place your mouth over the open end of the tube, and the other end near a piece of dust in the camera. Now inhale. To expel the dirt, turn your head to one side and blow. I would have thought this an ingenious device even if I hadn't been the one who invented it.