Have you seen this advice from this young monster? Like petting a cat, he says. I haven't been able to reproduce his results -- digging in always seems louder than petting the cat -- but he's also a much better player than I.
I have to experiment with this a little. I admit that I'm sceptical though. It seems to me that there's an optimum spot where one can get maximum volume without tension. Playing as lightly as possible (for me, anyhow) means turning up my amp, and that results in a less satisfying sound. Hmmm...