What is the best way to clean the inside of a DSLR camera body?

Unless its really bothering you I suggest just ignoring it. If you absolutely positively find its just too annoying to ignore then hold the camera with the lens mount facing down and use a small bulb blower to dislodge the dust. DO NOT touch the mirror because its a front-surface mirror and rather delicate.

When changing lenses keep the camera pointed down and avoid windy, dusty conditions.

You should avoid touching any part inside the camera body. Use a blower to blow away any dust accumulated. You can get this from any camera accessories shop. You can also use a compressed air aerosol if you have one. This is more powerful and you dislodge some stubborn dust sticking to the inside. When you use the blower, hold your camera face down so that the dust will fall away from the camera. Do not use your mouth to blow.

After blowing, use a very soft brush (the type use for camera cleaning) to brush the inside. DO NOT brush the mirror or the prism (any glass or reflective surface). Blow again.

This will be successful 99% of the time. If there are still dust, I would recommend that you send the body in to the service centre for cleaning.

Advice: When changing lens, face the camera body downwards. Change lens in a non-dusty environment. Prevention is always easier than cure.

OK...

compressed air: The best way to mess up a camera

bulbs with brushes on them. Cheap chinese pighair often, tends to let go whenever inconvenient. Too hard to touch sensitive cameras

Rocket blower: The best way for most people to clean camera insides.

I'd ignore anything on the mirror. It doesn't affect picture quality one bit and it will not show up on your viewfinder.

The coating on the mirror is far more delicate than the low pass filter on the sensor itself so be careful with it. If it bothers you that much use a bulb blower.

NEVER use canned air on a camera. The best way to clean it is to have Nikon do it. They are trained and won't harm it.

Use a 100PSI air compressor.

lol JK.

Have your local camera shop clean it or a trained professional.

or if you have skill there are cleaning kits for the inside of your dslr available.

The best way??? Find a pro.

they do have cleaners for those, which is basically a little bulb for air and a soft brush. I would suggest getting one of those

Pay someone more knowledgeable with these things.

have nikon do it for you.