Here I'll focus on aspects related to timing. (I'm unconvinced that reviews that take longer to get back are any better in terms of feedback. Often, just the opposite -- the reviewer realizes the paper has been on their desk too long and just writes a quick paragraph to be done with it.) Let me suggest a set of basic pledges -- rules that everyone should follow. Feel free to suggest your own additions....
- When asked to review a paper, I will respond as quickly as possible, even if the answer is no.
- I will review at least 1 journal paper for every journal paper I submit. (This does not mean "I will give my graduate students 1 paper to review for every journal paper I submit." Though graduate students should also do reviewing...)
- I will skim through a paper I plan to review within one month of receiving it, so that if it is a clear reject for any reason, or there is an important question that needs to be raised, I can promptly write a short note to the editor (and thereby hopefully avoid a more detailed review).
- I pledge to review papers within six months of receiving them. I acknowledge that six months is an upper bound, not a target deadline.
- I accept that there is reviewing karma, and I should not be surprised if editors pass my papers on to slow reviewers if I am a slow reviewer.