Please refer to other posts for instructions on how to remove the LCD mask.
I have found that using an air purifier, placed next to the computer during disassembly, cleaning, and reassembly is the best way to prevent dust particles from landing on the panel while cleaning it.
Using a Holmes Harmony purifier (type with a motor and HEPA filter) with the "exhaust" vent blowing air over the computer prevented dust from falling on the LCD mask and actual glass LCD during my cleaning operation. I also used a microfiber cloth and non-abrasive glass cleaner. Now, the M200 is cleaner than NEW!!!
Before this, I've tried canned air, an actual powered air compressor, and my lungs to blow off dust just before reassembling the panel. The aforementioned process using the purifier (on the fastest blower setting) is far better than anything else I've tried.