Get ready. Prevent difficult conversations from becoming emotionally charged. Ask yourself 'What do I want to achieve here?' before you go into the conversation. The answer can act as a reminder to pull back from an argumentative stance.
Start with the facts. Sharing your feelings is a powerful way to express why something is important to you, but differentiate between facts (the report has three errors in it), assumptions (it was clearly done at the last minute) and emotions (I feel let down). Facts are indisputable, so are easier to share first.
Describe actual behaviours. If delivering constructive criticism, avoid the infamous 'feedback sandwich' (good-bad-good). It comes across as disingenuous and dilutes the impact of your message.