If feasible, give feedback immediately. It's most effective when an exchange between individuals is fresh and clear in their memories, not clouded by ensuing events. Give people the opportunity and the time to incorporate your feedback in a constructive way without directly demanding that they change. Otherwise, they might be unwilling to listen further and become less open to changing their behavior. If you're sensitive to their need for time to consider your feedback, a positive outcome is more likely.
Feedback is most effective when people request it.
This indicates they are prepared to listen to you and may be willing to change their behavior. Often your feedback will elicit feedback from the other person. Be careful that such an exchange does not escalate into a contest to determine who can be more clever or hurtful with words. If a person reacts defensively or angrily to feedback, check to see if you've followed the above guidelines. That will help you evaluate how you gave the feedback and give you an opportunity to improve your delivery.