On a successful 3-point attempt, my understanding is that one is not supposed to bring the fist down, but use the usual "field goal" signal instead. (I've had this call a handful of times since I started reffing---one just thet other night, in fact. Of course, I brought my fist down out of habit...)

simple and to the point
 

Like JRut said earlier, business like. Don't be flamboyant. The best explanation as to how and why to do this is like this. The offended team scored and gets a chance to score again, the offending team already is feeling the effects, so don't rub salt in the wound. This day and age the offending team and fans can very easily take this as Showboating against them......I don't know who said it but it works for me...

Now my serious reply...

I always signal a foul with my left fist up (no particular reason, just the way I do it). However, being right handed, it doesn't feel right to cout the basket with that hand...it just feels awkward. So, I signal the basket with the right hand, while dropping the left. It looks sort of like a swim stroke movement with the right hand coming up about shoulder high then forward and slightly down in a fist (somewhat hammer/punch-like) as I take a step in with my left foot.

I like the captain hook.