since it is required that you turn on the signal few second before you take the turn, so mechanical auto signaling system (lol) where it automatic turn on signal when you turn the steering is out of question.
anther way is to have a gps system that plot the route you are going to take and then automatic turn on the signal when you are suppose to turn. it might be a good thing to both signal other people that you are about to turn, and probably good for reminding you that you need to take a the junction. but not everyone will plot their route before they set out for a trip, and someone might just be plainly drive around for fun.
another way is to have some system that read the user's mind. i don't know how this can be done, but if it does, it will be the same as normal signaling already, minus the switch flicking. but that would be a big problem for me since i'm use to flick the switch without thinking, and won't work for those last minute decide to turn cases.
one good thing i can think of this is probably make a sensor at the front wheel that detects whether you are going across the white line in the middle of the lane or not. where it will automatically turn on the signal. late, i know, but better late than never what... this i think will be better than what is mentioned above, since i'm lazy to use the signal when i zig zag through the traffic. tried that, but sometime its just hard when you are driving very fast.
