I would put the socket closer to the center of the bike to keep the wire movement to a minimum, for one, but moreso...

Mostly, I'd mount it horizontally (actually ends up pointing slightly downward) to have a better chance to keep the rain out of the socket. I haven't shorted mine out yet in the rain, but I unplugged everything and closed the rubber cap. Not a bad idea, turning off all the goodies makes it easier to concentrate on keeping your @#%$ together, plus they are all less likely to short.

Also, if I were going to switch anything, I'd probably do a relay and switch it with the bike instead of putting in a rocker. However, sitting there on the bike with the radio on, and the key off is sometimes advantageous. Just make sure the switch is a marine grade (waterproof) switch.
Matt
www.DocThrock.com