1. Is the feedback specific and actionable?
There must be a call to action and you'll want to be sure the designer can follow through on most or all of the suggestions you make. Also, make sure to differentiate between changes you want now, and what you consider to be future changes -- don't make the designer guess. Deadlines and timeframes should be reasonable enough to allow for the changes you're requesting.
2. Is the feedback contextual?
The designer should be able to quickly identify what your suggestions mean. Notable is a great option for putting feedback directly on a screenshot. Alternatively a print out with hand written notes works well too.
3. Does the feedback encourage your team?
Tear down any roadblock so the team feels empowered to get results. Get people excited about your insights and save any cutting or extremely negative remarks for a private conversation. But don't sugar coat mistakes or problems.
4. Is the feedback within the recipient's scope of skills?
Whenever possible, break down the feedback and expected actions into smaller, obtainable chunks. This will expose any potential challenges that occur when the changes you request don't match the designer's skill set. Remember, just because it needs to get done, doesn't mean the person you're talking to is the one to do it.