Are the crosshairs moving because the rifle is moving, or are the crosshairs moving because your head/eye is moving and you are seeing the Paralax in the scope?
What position are you shooting from? I'm not great at it, but what I've read, bone on bone contact eliminates muscles from the hold and limits the influence of your heartbeat. If you are shooting from the bench, you might be squeezing the rifle too tight. What rifle are you shooting?
Other things I've heard, aspirin (thinner blood), meditation (slower heartbeat), thicker clothing, gloves, different position.
AJ