What I found really helped was actually sanding the tips slightly with very fine (I used #800) sand paper. It made a world of difference!
This is unneeded, the frequent use of the test leads over the PCB soldering, keeps them always free of corrosion but eventually it takes some of their life cycle away too.
One iron sponge for dish washing with some oil, could be a safer approach about cleaning the probe tips from any oxidation.
The nickel plating is just a thin soft layer that protects the copper under neath from getting green or black,
It must be handled with care.