I may try Jobob's trick of "waving", but actually I do a bit like your Tom guy but I don't point (which is indeed perhaps a bit agressive). Instead, I stretch my arm and put my hand in a "stop" motion (perpendicular to the ground) just like the traffic cops do at intersections (quite universal and drivers should be familiar with this).

I do that especially in front of drivers which I feel might not give me rightful priority when I am going straight and they are turning. Actually I read that most accidents on bikes are caused by drivers turning in front of a cyclist because they underestimate the speed of the bike and/or overestimate our capacity to brake. By signalling to them I draw their attention and protect my priority (and above all my life!!). It's kinda hard to work up if the car is coming from behind and wants to turn right, though, especially since the intentions of the driver are impossible to guess unless you have eyes on your jersey pockets.

I'll try to accentuate the waving and make sure I smile. I'm totally agreeing with the idea that we have to maintain peaceful relations with the drivers and other users of the road (including pedestrians). That's among the reasons why I always stop at red lights. I behave like a (responsible and respectful) car driver would as much as I can.