a) fenders - just keeping water (the high pressure spray from the wheels) out of key areas (downtube battery/controller) really helps
b) plastic covers for occasional use - split ziplock bag over the display, or a larger form fitting plastic bag over the battery are used only during heavy rain.
c) sockets/connectors treated with dielectric grease - this helps to keep dirt and moisture out and maintains good electrical connections
d) black tape around "waterproof" connectors - the extra layer of protection seems to help and it's a throwaway, it gets replaced any time the connectors are opened.
e) in the fall I start to use higher viscosity lubricants on any moving parts. The slightly heavier/stickier lubricants seem to do a better job in wetter conditions. Spring time brings more cleaning and lighter and sometimes "drier" (wax based) lubricants.
f) the fall cleaning, relubing, and connector checking ... a deeper overall preparation for the rainy season. A good cleaning helps to catch issues with cables and connectors (looking for splits, cracks, wear, etc.). Re lubing as mentioned above. The connector checking is pulling off any tape, opening connectors and re applying the dielectric grease and tape.