The Give and Take of Criticism

Giving constructive criticism is tricky.  We can hesitate to criticize because we are intimidated to do so or wrestle with feeling “judge not lest you be judged”.  Or we can slide over into the other ditch and become bold and too matter of fact with criticism, becoming blunt pushing too many buttons with results that are less than positive. Both responses are unhealthy. How do you give constructive criticism without being a jerk? How do you constructively criticize someone’s work or behaviour without it coming across as a personal attack?