What I do with my mirror finish shield is to wet a paper towel and stick it on the shield for about 5 minutes and you don't have to do any scrubbing... just use the same paper towel to do the initial cleaning and use a dry paper towel to dry/clean the shield. Works great for bug splatters. I've used this method with my shields and they last a good year or so until I have to replace because of pits and stuff from road debris. I haven't had any problems with the coating coming off from this method.
I like helmets... It saved my butt... uh... face a few times. This is one of those times.