I always use a right-turn arm signal if there are other vehicles around, and usually a left-turn signal, though I sometimes deliberately don't signal left if I have a feeling that a driver might use it as an opportunity to cut me up (both from in front or behind).
I have some other improvised signals, don't know if they make any difference, but they usually seem to have the desired effect. In some multi-lane situations (e.g. when going straight on with a left-turn only lane to the left), I will vigorously point in my desired direction of travel. Sometimes, when it seems like a motorist is about to overtake too close I will use a sort of backward stop signal, or some more vigorous pointing gestures, trying to signal 'stay there!'. If that works then I will make a show of waving them past when there is space to overtake safely.