I usually use isopropyl alcohol and a q-tip to remove what I suppose is the silver oxide on the contacts. Sometimes the file or flexstone is employed, especially if the contacts are pitted or otherwise not mating well.
And always operate the switch stack to ensure proper opening/closing. The switches should not just touch, but bend a bit when they are closed. Sometimes the amount of travel in the relay or mechanism is small and it is a finicky job to get them properly adjusted.
The joy of an EM is that a single wonky contact can make things not work!