For a spinner, use a good quality one with a metal backplate rather than one of those plastic ones (the plastic "gives" too much, allowing the nut to loosen).

When you are not using a spinner, the depth of the prop on the shaft may mean that you are running out of thread before you are really snugged up against the prop properly, so check for that.

Other than that - snug it up good & tight, and make sure your mixture is set correctly. Backfiring is a common cause of prop-nuts coming loose on 4-strokes, but can also happen on 2-strokes as well.

Good luck
Gordon