I am just finishing my first year in a tenure-track position, and over the year I have been invited to review a lot of papers. Some weeks I was getting 2-3 requests. In the 10 months since I started, I have received at least 50 requests. I was able to eliminate some because the paper was outside my area, or the author was a friend or collaborator. But, most of the papers were interesting, in my area, and would be nice to read and provide feedback before they are published. Moreover, the editors inviting me are often supervisors, past collaborators, or potential future collaborators; all whom I would like to maintain good relationships with. If I said yes to all of them, I would become a professional peer reviewer, and have no time for anything.
What is a reasonable number of peer-reviews to do in a year? Does anyone have strategies or techniques for selecting which papers they review, and how to decline review-requests politely?