The one I heard from my mother
Be good. If you can't be good be careful.

And the version I heard from my daughter
... If you can't be careful, buy a pram.

The one quoted above about skydiving is from Les Barker. Who twists several sayings in one of his monologues.

Can't find that one online so how about this one of his
A boomerang's not just for Christmas;
A boomerang is for life.