For many years I've used my treasured little bottles of Cramolin Red and
Blue. The Red is the cleaner and the Blue the preservative. They're a tad
pricey but you apply them with a toothpick; for most hobbyists a small
quantity lasts a substantial fraction of a lifetime. Cramolin Blue on new
contacts and such keeps them working like new. Cramolin Red is what you
want to clean up grungy card edges and connectors, and those relay
contacts.
This is the good stuff. (No, I don't work for those guys.)
These products are now made by Caig Laboratories and they've changed the
names several times over the years. I think what used to be Cramolin Red
(cleaner) is now called Deoxit, and Cramolin Blue (preservative) is now
Deoxit Shield, and they may not even be red and blue any more (sacrilege!).