If you have kids—or have ever seen one—you know that something like “Put that down!” will lead to “Put that down. Now!” which leads to “Put that down, on the table, now!” which leads to “Put that down, on the table, gently, now!” Specificity works as well with adults as it does with kids. Maybe better, since many of the people around you are more conscious about their growth and development than little kids.
Be specific about what the person did, why it was good or bad and, if necessary, suggestions for how to do better the next time out. Timely, accurate feedback is a gift. Really.
When you tell me the truth my performance I know exactly what to keep doing, stop doing, or do differently.