Tyler Cowen has some tips here. I disagree with his point 2. I try to do all referee reports within 15 minutes of receiving them. On the other hand, it would probably be a disaster if all referees followed my approach. A diversity of individual strategies probably results in the best collective outcome. I’m often impressed by the elaborate referee reports given for my own articles. On the other hand, my reports are always on time, and my judgments are trustworthy (I think).
Also, my impression is that the referee process is more serious in economics than in other fields, so that might explain some of our differences in approach.