I've become addicted to a simple new app by Katrina Owen.
It is called Exercism and I have found it to be a really great way to practice both giving and receiving code reviews.
It works like this: you are given a small programming problem with a suite of unit tests. You make the tests pass and submit your solution for critiquing. You have a few cycles of feedback — people that have completed the exercise make 'nitpicks', you make some changes, repeat until enough people mark your answer as completed.
Unlike the more mathematical Project Euler or the fun-but-impractical code golf challenges, the aim of Exercism is to help you write the most "simple, expressive, readable code". So, you know, actually useful in the real world.
As I've mentioned before, I think it is really tough to get good at code reviews. Coming out of school, the only code review you have probably received was from an automated grading script (and maybe a 5pt deduction from the TA if your source code was a mess). And I have yet to find a good resource to help guide an experienced developer on how to give constructive feedback to a newbie.
This is an area where Exercism really shines.
To be able to nitpick a submission, you first need to complete the exercise yourself. This ensures that users see at least one example of how to conduct a review. You are also encouraged to nitpick your own submission and the app shows you guidelines for what is appropriate:
Once you finish an exercise, you can spend your time reviewing other submissions, giving you a great opportunity to practice doing small-scale code reviews. Since you have already solved the problem, even if you are a relative newbie, you can still provide feedback (even if you are just parroting back the feedback someone else gave you).
As a veteran developer, Exercism is a great way to practice giving polite and thoughtful feedback to a captive newbie. As a new developer, Exercism allows you to gain experience receiving criticism, asking for clarification, and determining when to take (or ignore) suggestions in a code review.
The app is still in the experimental phase but I would highly, highly recommend you try it out. I've gotten more practice at code reviews in 15 minutes a day on Exercism than I could have in several weeks at work.