Great questions! I believe project teams that embrace feedback perform better.
At the team level, there is the Retrospective. This occurs every other week (at the end of an iteration for us) and provides an opportunity to review what has gone well and where we have opportunity. These meetings require some time before they are beneficial if the team is new or if an existing team is experimenting with it for the first time. If you are in one of the leadership positions (Dev Lead, Test Lead, Requirements Lead), I encourage you to offer positive, constructive examples of the good and the challenging. It may take some effort over a period of time to have everyone participate; stick with it, it will be worthwhile!
I also encourage personal feedback. When you are considering feedback for a team member, consider the person and the place. Discussing opportunities for improvement are best explored privately.
I agree with George. Specific examples are necessary when providing feedback - especially if there is an opportunity for improvement. Also, focus on a behavior. For example, for someone who is struggling with writing test cases, pick a few of the weak test cases and talk about them. Provide an opinion on why you believe they are weak, suggestions for improvement and why the improvement makes them stronger. When delivered in this manner, you express both support and confidence in their ability to improve. I strongly recommend the book “Crucial Conversations” for both simple and tough feedback. The Vital Smarts blog sometimes has great examples of delivering tough feedback.
Positive feedback is, as you might imagine, easier to deliver but requires the same consideration: focus on behavior, outcomes, or actions. This has a spectrum from a simple “Great job on those test cases!” to sending positive feedback to them through email and copying their manager. You may want to follow the STAR (Situation, Task, Action, Result) format.
My department also encourages and supports more formal feedback through public recommendation for recognition. An internal website collects information (in a STAR format) which is reviewed by a committee. The committee rewards individuals and teams a bonus. Additionally, the same system is used to publicly thank someone for a extra effort. When I use that system to thank someone, I wait until Friday - great way to start a weekend by being recognized for their efforts!