Well, it is hard to examplify it but for me it is:
- Should not only depend on quantific things such as completing x- quests or helping to reach x level. Maybe some sort of special designed dungeon can be open for a duo of high and low level, or some visual/video materials can be prepared in forums, or maybe ( a bit extreme?) some video material can only be reachable in game for apprentices. With some sort of voting it can be evaluated.
- There should be unique rewards which will attract both players for it
- A bullettin or etc. may be available for who seek master's or apprentices
- It should not be spammable and open to exploit for people with alts.
-People who are genuinely help in a significant frequency should be honored with a title or etc.