I find there are four main sources of stuff on the lenses.
- Fog/condensation from temperature changes
While riding one can temporarily remove rainwater by wiping the front with an index finger, from nose to outside, one lens at a time. Rainwater is generally clean so low risk of scratching. Reduce incoming rain using a cycling cap/casket aka a visor/brim on your helmet.
For sweat, it is salty and will leave deposits behind if you wipe. A sharp "nod" motion helps sweat to drop off the bottom lip, and any sweat/salt is mostly out of the line of sight. You can also help divert sweat with a headband or similar.
Fog forms for me when riding hard in the cold and then stopping at lights. Wiping doesn't help here cos the fog is on the inside surface, and trying to wipe results in awkwardness and the risk of dropping them. Instead, pull the frames down the nose a bit to let the cold air in easier which helps equalise the temperatures. As soon as you start to move, the cold air clears the fog though this can be a bad time to have impaired vision.
Dirt risks being abrasive - while riding its best to stop and splash some drinking water on the lens. Don't wipe dirt/mud off with a glove.
After riding I've been told that any chemicals/cleaners are bad specifically for the frame that holds the lenses. As such, I wash them off under a tap or in the shower. The three pieces are separated for easier access.
Shower-temperature water has not caused me any problems in the past, but the inner frames are not very durable anyway and show cracks within 6 months. I have bought plenty of spares and figured out a safe way to insert the lenses (involves one's mouth, teeth and lips have a fine degree of sensitivity for the pressure one can exert !)
Dry with a clean cloth or paper towel in a patting motion, not a wiping action.
Edit - just after writing this, my inner frame let-go again.
This happens every 6-24 months depending on how much riding I do, but is unavoidable. A crack is visible through the clear plastic, and after some months it breaks completely.
I've managed to move the lenses to a replacement inner frame using a combination of firm-gentle pressure and luck, and I buy the inner frames from Aliexpress for a few dollars.
Rest of frame, for context.
The prescription lenses cost twenty-times what the frames did, so they're the part that needs protection.