I use canned compressed air to shift the big bits, and any smears or marks such as water splashes etc I use the cleaner you get from Boots for specs. That so far (after many years) has never damaged the coating on my glasses.
When the pump spray is empty, you can refil it for about 10p with exactly what they charge over £5 for. A 50cc pump bottle needs 40cc distilled or ionised water, 5cc wetting agent and 5cc isoproply alcohol. I have all the above so it costs me nothing.