I started my own photocopier repair business when I was 19. I did that for about 8 years.
Most "repairs" actually just involved cleaning due to dirty optics.
After trying many different products and techniques, this method is what works best:
1) clean thoroughly with a paper towel saturated with rubbing alcohol to remove any dirt or grime.
2) quick wipe down with a wet towel or cloth to remove any residue and to wet the glass.
3) buff dry with a clean, dry towel to remove/prevent streaks (terrycloth towel or rag works best).
Most glass cleaning products will leave streaks or residue that may take a week or two to show up, especially ammonia based.
The above method works great every time and will leave the glass clear and streak free.
