Casting in a park by yourself can be a bit embarrassing, yet if there are two or three of you that feeling goes away
I think the trouble with a bit of practice on the river is you'll inevitably end up fishing
I think for most of us, just thinking about your casting and trying to improve technique while you are fishing can help?
There is no doubt that an hour or two with an instructor once a year is a good idea, but how many of us do that?