Scrub with soap and water, rinse, then bake dry. Some people advocate
running them through the dishwasher. Non-potted transformers and
relays may not like getting wet.
Final rinse with distilled water to assure no chemical or mineral residue
from the tap water. Either bake dry or let air dry for at least three days.
Then follow up with a good inspection before attempting to implement use.
Make sure there are no residue carbon (Smoke & Soot) traces between
components that can produce a resistor like function with application of
current.
Use a dishwasher detergent WITHOUT "rinse aid", and as someone
else already suggested, final rinse, by hand, with distilled or
deionized water and blow dry.
Watch out for non-hermetically sealed components of all kinds;
non-potted transformers, relays -- but also switches, jumper
blocks, connectors, keypads (&c &c)