When you do provide feedback, start by asking the person their opinion on the situation. Then, point out where your opinions are similar. If, at that point, there are still differences between how you and the other person perceive the issue, point them out.
Remember to balance the feedback with positive reinforcement—many psychologists believe positive reinforcement is much more effective than negative.
SOURCE: "101 Ways to Have a Great Day @ Work," Stephanie Goddard Davidson, 1998.