Greetings from Australia

[Sanhestar]

I use a tongue click - many clicker trainers do - this sound that you can make when you put your tongue against the roof of your mouth and then let it "snap" down. Is less invasive especially when you train with other people around you and, as you said, you always have both hands free. And it's always with you.

I'll copy the post and elaborate a bit more. Will take some time, though.