Only ever really done this in terms of ending counselling/mentor relationships.
Advice to us was to prepare the other person in advance, i.e. explain that the course of counselling is due to end soon. It's a good time to review the relationship, talk about the highs and lows, what has been achieved and to identify how the other person will move on from here. In short, don't just end suddenly and leave the other person hanging. It was also suggested, in a mentoring context, that the end of the relationship be 'celebrated' with something special, perhaps a shopping trip, cream cakes or something similar.