Here are my tips:
1. Assume that no referee reports are truly anonymous. It is fine to be critical but always be polite.
2. Unless it is immediate junk, read the paper once and return to it a week later with deeper thoughts and a fresh mind.
3. Your report should not assume that the editor has a working knowledge of the paper in his mind.
4. Respond within a month. First it is considerate to the author. Second, the less "fresh" the task is, the more painful it will be.
5. A properly critical and useful "accept" report is harder to write. Don’t look for excuses to quickly reject a potentially good paper.
6. The editor might have chosen you as referee for a reason. You need not go along with the editor’s grand plan or desired outcome, but be aware it may exist.
7. Don’t check the references to see if you are cited.
Barkley Rosser, a frequent MR commentator who has sent me JEBO papers to referee, may comment on this post but not on my adherence to these standards, especially #4.