Great feedback happens when you pay attention. Close the door when you offer feedback, either literally or symbolically. If you don't have a door, make a gesture that signifies you're giving undivided attention to the person. For instance, clear papers off your desk, stop whatever task you were engaged in or move your chair closer. The Quick Draw.
Great feedback is immediate.
If you notice the feedback is important to someone, don't hesitate. Prompt feedback helps establish a link in the other person's mind between her action and your response. Let's say you notice an employee struggling with some paperwork. She appears to want help. I've noticed that you seem to be struggling with that paperwork.