Code review becomes very essential part of the development and makes our code up to standards. There are couple of things that reviewer should always keep in mind while doing code review

1) Questions: don't make demands.

     i) what do you think about naming this method into this?
     ii) I think changing our logic will improve performance what do you think?
     iii) Let's not use aberrations, are you agree with that?

2)  Avoid using terms that could be seen as referring to personal traits. ("dumb", "stupid").   Assume everyone is attractive, intelligent, and well-meaning.

3) Avoid selective ownership of code (mine, yours or not mine etc.) Using terms like ours, us looks more sophisticated.

4) Humbleness: show humbleness in your words as well
     I'm not sure - let's look it up.
   Instead of
    I don't know, you should check it

5)  Don't use sarcasm & don't use hyperbole. ("always", "never", "endlessly", "nothing")

6)  Accept that many programming decisions are opinions. Discuss tradeoffs, which you prefer, and reach a resolution quickly.    

7) Be explicit and give examples if possible. Remember people don't always understand your intentions online