ClearBoard is just like our eyes. If you cannot read the board easily, ClearBoard would also find it difficult.
Write clearly with high contrasting colors. Try avoid colors that appear very faint on the board. Don't write too fast so your writing appears more visible. You can try to increase the contrast of your image by setting a higher contrast value.
Write big. How small you can write depends on the resolution of your camera. The higher the resolution of your camera is, the smaller you can write on your board. For most digital cameras with around 1 Mega pixels, the rule of thumb is that your writing should be easily readable by a person who is around 3 meters away from the board.
Keep your board clean
Cleaning your board throughly before writing can always help. Some people like to hang some small objects on their boards. Remove those objects if you can.