From the optician I use microfibre cloth for glasses. Also have a optical wet-wipes, brush and camera lens paper in my backpack. Swarovski does a really nice lens cleaning kit and refill packages. I have one of these in a waterproof in the backpack and refill with cheaper supplies from the optician.
Considering how much a quality scope costs, it's kinda daft to ruin it with toilet paper or shirt ends.