It is almost impossible to clean the contacts by chemical means alone, the toothbrush will help. In really grotty cases I use a small "swiss" file, you can buy small file kits relatively cheaply. If you are tempted to chop the connector off and resolder a new one on, be careful, because you may find the wire itself has a small amount of oxidation and will not solder easily, although a bit of fine sandpaper may suffice to clean it.
To prevent vergris in the 1st place, i reckon the spray silicon grease that plumbers use is as good as anything, because it doesn't dry, doesn't rot plastics, is water repellent and you can really lather it on.
It is also possible to buy switch contact cleaner, it does help, but not in the really grotty connectors.
I find WD40 a tad overrated IMO, discuss.....
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