Google's take on code reviews is excellent
If you write code with others for any reason (work, open source, volunteering, etc.) and are reading this page, stop what you’re doing and read this document.
My favourite parts of the document are quoted below:
If you see something nice in the CL, tell the developer, especially when they addressed one of your comments in a great way. Code reviews often just focus on mistakes, but they should offer encouragement and appreciation for good practices, as well. It’s sometimes even more valuable, in terms of mentoring, to tell a developer what they did right than to tell them what they did wrong.
One business day is the maximum time it should take to respond to a code review request (i.e. first thing the next morning).
There is one time where the consideration of personal velocity trumps team velocity. If you are in the middle of a focused task, such as writing code, don’t interrupt yourself to do a code review. Research has shown that it can take a long time for a developer to get back into a smooth flow of development after being interrupted. So interrupting yourself while coding is actually more expensive to the team than making another developer wait a bit for a code review.
In general, it is important to be courteous and respectful while also being very clear and helpful to the developer whose code you are reviewing. One way to do this is to be sure that you are always making comments about the code and never making comments about the developer. You don’t always have to follow this practice, but you should definitely use it when saying something that might otherwise be upsetting or contentious. For example: Bad: “Why did you use threads here when there’s obviously no benefit to be gained from concurrency?” Good: “The concurrency model here is adding complexity to the system without any actual performance benefit that I can see. Because there’s no performance benefit, it’s best for this code to be single-threaded instead of using multiple threads.”
I also found this related document on emergencies an interesting read.