how to clean a dslr mirror

Are you asking?

For light cleaning you can use a lens cloth. For moderate cleaning use a cotton swab (on a stick, commonly known as a Q-Tip) with some lens cleaning solution (perhaps any mild glass cleaner would also work). Put a drop on the end of the cotton swab and clean the mirror. Once the mirror is clean, use a dry cotton swab to remove any excess liquid. For heavy smudges you can try using some lighter fluid. It should not have any negative effect on the plastic parts in the mirror box but use sparingly so you avoid getting the liquid everywhere. I use lighter fluid on small cotton rags to clean areas where there are light seals when replacing the light seals, including the mirror bumper.

Peter

Since the mirror doesn't affect the exposure, I tend to let mine get a little dirty, but when mine got really bad, I used a sensor cleaning swab and sensor solution in more or less the same way you'd do a sensor.  Those wide swabs are basically all done for sensor cleaning after one use, but they're still nice and non-abrasive and cleaner than almost anything else.  You can probably use just about anything that's reasonably volatile and harmless, alcohol or lighter fluid, but sensor solution is also pretty good, and it's designed not to leave streaks.

You do have to be careful with this mirror because, as you undoubtedly already know, it's a front-surface mirror, which is delicate and not repairable. So whatever you do, do it very lightly, because if dirt is left you can do it again, but if it's scratched, it's forever.

Are you sure you need to clean the mirror?  It is very delicate, and does not affect the image.  I don't think I have ever cleaned the mirror outside of using a blower or a sensor brush.

I agree with the above about not cleaning the mirror. What does it have on it?  It is easier to scratch a mirror than a sensor, and as noted above anything on the mirror has no effect on the image produced. I've used DSLR's since early 2000's and never cleaned a mirror.

As others have said…no need to. If you damage the mechanism, that flips the mirror up, kiss a lot of money goodbye. While a spec on the mirror can be, annoying, it won’t affect the image. Hopefully a blower or lens brush will take care of the spec. 

I agree that unless your mirror is really filthy, there's little reason to clean it.  In my case after nearly a decade of use including things like changing lenses in bad weather and the like, mine had gotten pretty nasty, to the point where some water stains and particles were hard to separate from the image, and I did find that a very careful cleaning with a sensor swab worked.  But unless the junk on it is intrusive, I'd let it be.

Posted by: @formerly-bruto

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Since the mirror doesn't affect the exposure

Since the focus and exposure sensors only get light passing through the mirror I’d say it does a bit affect it.

The focus and exposure sensors are enormous in comparison (to make up for the relatively tiny pickoff through the mirror). You would need a pretty big smear for it to matter even imperceptibly.