For a light microscope in bright-field mode I follow the following methods.
1. The sample needs to clean and properly stained.
2. Chose the right objective/magnification to suit your purpose.
3. Always do a background substraction prior to imaging to remove any marks in the optical path.
4. The sample needs to be properly illuminated with sufficient light..and not underlit or saturated.
5. White balancing needs to be performed. White balancing can sometimes be avoided if the illumination source is white light ( LED) but is compulsory for yellow light sources ( halogen).
Ofcourse there are more advanced methods to improve the image quality more..but these are the absolute basic.