SBI stands for "Situation, Behavior, Impact" and it is the best tool I've found for giving people constructive feedback.
When did this happen? Who was there? Where did it happen? The more precise you can be, the better.
Describe what you observed. Focus on the behavior, not the topic or idea. Don't use SBI to provide feedback on ideas.
How did this behavior impact you (personally).
How did this behavior impact other people?
How did this behavior impact the organization or project?
Reply to this thread with some SBI feedback on my Twitter usage.