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Q-Tip swabs and isopropyl alcohol works well if the eyepieces are not very dirty. Zeiss and Nikon lens cleaners are basically that.
If the eyepieces have mascara or grease or fingerprints (shudder) on them, then acetone can work well, as can lens cleaning fluids that contain a trace of detergent.
Use 1-2 drops on the swab--never spray directly on the lens.
I make a spiral pass from center to edge with the wet swab, then flip it over to the dry end to mop up. Grab a second swab and, moving from edge to just past center, move the swab back and forth in a straight line while turning the eyepiece under the swab. Keep this going until you've done two revolutions of the eyepiece.
If there are any left-over streaks on the eyepiece, repeat the procedure using pure distilled water to remove any residue.
NEVER, EVER wipe a dry lens with a dry cloth or tissue until the eyepiece has been cleaned with a fluid and there is nothing on the lens. Coatings are hard, but not invulnerable.
As to frequency, I clean them as often as I see debris on the lenses--usually once every month or two. I notice that the eyepieces I use with glasses on never seem to need cleaning. One of the advantages of needing glasses at the eyepiece. Glasses also keep the eyepieces from fogging up.
As to the frequency of cleaning glasses, well, anyone that wears them knows the answer to that...
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