It depends on how nice/expensive the glass is. The prefered way for the most expensive stuff (NV/Thermal) is this:
1. Spray the lens from a glancing angle with a can of compressed air to remove larger pieces of debris. If you go straight into using a lens cloth, you will inadvertantly rub microscopic particles into the glass coating, scratching it.
2. Use a soft brush (like the one that comes in a lens pen) to lightly go around the edges to loosen up any debris that is caught in seam between the glass and the housing. Spray with air again if needed.
3. Use a lens cloth or lens pen to gently wipe out smudges and finger prints starting in the center and working your way out towards the edges. Use ting circular motions.