Objectivity is crucial to constructive criticism.

The goal is to communicate that a performance standard has not been met. Your sentiments/judgments are irrelevant.

When preparing to tell someone what they did wrong, avoid using qualifications like "With all due respect," "No offense," or "Don't take this the wrong way" to soften criticism.

... and if you must amplify your message, say where your data came from. Never try to simultaneously be a good cop and a bad cop. 

Make it clear that your goal is constructive change.

Criticism without an action plan is worthless. 

Give people direction or keep your mouth shut.

For example, in saying "You were lazy in preparing this report" you may think you are helping the other person to improve a skills; instead, it addresses your assumption about the person's attitude toward their work.