No relay required.
From the battery positive terminal place an in-line fuse to a luminated switch then to positive wire on the power sockets/USB. This way the power sockets energised without the ignition on, I prefer this way, myself.
Negative wire from power sockets/USB to battery negative.
Power socket/USB and luminated switch lives in the nearside glove compartment.
It runs my Iphone, GPS, and spare Bikecomm bluetooth.
Will take photo if you like.
This is not a big job to do, and you can get your parts from Jaycar/Altronics/Ebay etc.
Cheers
Dave