Things to watch out for

assume good faith and competence, question with curiosity
discuss and iterate on overall design before discussing details in the code
if you foresee the review will take multiple phases, announce this to the submitter upfront to set expectations about process
review code for style, prefix "nit:" for small style recommendations
review the commit message: motivation alongside what was changed
check and double-check the release note: user-facing changes must be mentioned, and backward-incompatible change must be highlighted

The human side of things

Intro:

More detailed guidelines / background:

Science:

Bacchelli, A; Bird, C. Expectations, outcomes, and challenges of modern code review. Proceedings of the 35th IEEE/ACM International Conference On Software Engineering (ICSE 2013).
Beller, M; Bacchelli, A; Zaidman, A; Juergens, E. Modern code reviews in open-source projects: which problems do they fix?. Proceedings of the 11th Working Conference on Mining Software Repositories (MSR 2014).