I bought one of these, it works really well:Exactly how it should be done, using normal hand signals, the indicators are used to 'be seen' and confirm your direction when you have to put both hands on the handlebars to complete the turn. Talking about mirrors... i've found most mirrors on the market are too short for these ebikes and only show your arm in them. I've had to cut mine and fit an extension piece in the middle, perfect now!
http://www.merlincycles.com/cycle-aware-reflex-helmet-mirror-56440.html