To solder and desolder a very small things

[abdulbadii]

Would you share your golden tricks to be able masterly solder and desolder a very small pins (holes) tightly laid in a narrow PCB ?

[David Hess]

My first trick is lots of Kester 1544 or equivalent rosin flux brushed on.

[DaJMasta]

You're probably going to have to be more specific for particular applications, but some options to look into:
Hot air gun with a narrow nozzle
Tweezers with a bend
Inspection optics (magnifier, microscope, etc)
Good gel flux (at least for me)
Fine solder
Kapton tape to protect adjacent parts
Holder for the PCB

Sometimes even a preheater can help you get in and apply less heat from the topside to narrow spaces.