Do it often. Virtually no-one thinks they get enough feedback, because virtually no-one gives enough.
Give it promptly. This way, what happened is fresh in everybody's mind and it will be easier to learn from it.
Put it in context. Before you give feedback, provide the context it refers to. For example: 'I wanted to talk to you about the report you wrote yesterday.'
Be specific about what went well and what didn't for the individual or the team.
Avoid the 'feedback sandwich' (good-bad-good). It comes across as disingenuous and dilutes the impact of good feedback. Try to describe actual behaviours.
Focus on the impact of the event - what it was, and on whom - in order to explain why it is important.
Praise the individual's strengths. Feedback should motivate and there are few greater motivators than being told you're a wonderful person.
Positive feedback should outweigh the negative as it has a more predictable impact - tell people what they're doing well and they're likely to repeat it. The ratio of negative to positive feedback in primary schools is 19:1. And it isn't that different in most organisations.
Offer individuals the chance to respond, but let them reflect, and agree to talk about it again at a future date.
Find the right time and place. After giving critical feedback, make sure there is an agreed way to progress.
Don't let your personal prejudices get the better of you. Remember, you are giving feedback for the other person's benefit and not to vent your own spleen