Some suggestions for ensuring your feedback is constructive:

• Be gentle. As long as you get the message across, there is no need to be harsh. Receiving even nicely-worded negative feedback can be difficult. Put yourself in the recipient’s shoes and imagine how you would feel if you were on the receiving end of the message you just wrote.
• Be specific. Provide as many details as you can without deanonymizing yourself (should you wish to remain anonymous). Bad: “Your meetings are wasting people’s time.” Good: “When you arrange meetings, sometimes they are ineffective because there isn't a clear purpose and you don’t stop people who go off-topic.”
• Admire. Just as the recipient may need your feedback to change behavior that bothers you, they may not be aware of the good aspects of their behavior, and even if they are not blind to them you can encourage them further by giving explicit praise. Use Admonymous to encourage positive behavior as much as you use it to change negative behavior.
• Pick actionable things. Make sure the recipient can act upon your admonition. Even better, provide some suggested courses of action. Bad: "You're annoying." Good: "Some habits you have are annoying. It would help if you made fewer clicking noises, and let others speak more in conversations."
• Don't abuse. It's easy to be mean when anonymity protects you. Remember that the goal is helping the recipient, not making them feel bad.