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.