Like the shielding layers on sensors and filters, lens coatings scratch easily, so tread lightly when giving them the old spit-and-polish. In fact, go ahead and avoid both spit and polish. Start instead with a few blower puffs on both the interior and exterior lens, followed by some light sweeps with a soft camel-hair brush. And again, never blast your lenses with compressed air, which can leave behind damaging chemicals [sources: Delaney; Kelly; Stern; Weitz].
Leave cloths and papers out of it until you clear the area of dust, or you risk grinding grit into the lens, after which you'll dream of the halcyon days when smudges were your worst concern. If you decide to use a cleaner, don't go for over-the-counter options; instead buy one designed for your camera and lenses; otherwise, you might -- you guessed it -- damage the coating [sources: Delaney; Kelly; Stern; Weitz].
Removing smudges or fingerprints requires a delicate touch. Breathe lightly onto the lens to add a modicum of moisture, then take a microfiber cloth or lens paper -- folded, never scrunched up -- and wipe in gentle, circular motions [sources: Delaney; Kelly; Weitz]. Remember how, when you were a little kid, your mom would lick a napkin and scrub your face with it? That's exactly what you shouldn't do.
Out in the field without your gear? An old cotton T-shirt will do in a pinch, but never use a fabric or tissue with a rough texture, such as polyester rags, paper towels or facial tissue [source: Weitz]. Nikon's Lens Pen offers another option: One end has a dry brush for whisking away specks, while the other sports a soft round chamois tip with a non-liquid lens cleaner [source: Kelly].
As for cleaning the mirror ... don't. Just don't. Leave it alone. At most, give it a few puffs of air from your lens blower, but never touch it. The mirror's surface coating takes scratches like an iPhone in a blender, so either live with it or have a pro service it [source: Weitz].
Finally, if you're concerned about the camera shell, a wipe-down with a damp cloth will usually do the trick. Lens blowers are handy for blasting dirt out of nooks and crannies. In some cases, you might need to treat certain parts, such as leather straps, with extra care [source: Delaney].