I'm not sure how well any of these tricks actually work. The thing is, you need to concentrate to learn to fly, and we're all used to tensing up when we concentrate. It's worst of all when students are learning to hover, when "white knuckle syndrome" is well known.
The "trick", if you can do it, is to stop caring about how well you do. But since most pilots are high achievers, and most people are counting the money this is costing them, that's quite hard for many of us.
A few things which help, in no particular order, as it's late and I'm thinking as I write...
Take deep breaths - you'd be amazed how many people actually forget to breathe.
Consciously relax your grip when you remember, and eventually it will become a habit.
Don't beat yourself up about being tense; it makes matters worse.
If your instrucror's sexual innuendo type tricks don't work - they never did for me - tell him/her so, and try a different way.
I chat to students...too much, probably, but it seems to stop them thinking about hovering and then they start to relax.
Hold yoke/stick/cyclic with fingers and thumbs, not whole hand - it makes it much harder to grip.
Then take a few more deep breaths.
And if it still hasn't worked, you're normal, and practice makes perfect, so just carry on and trust that the process of training works.
