How to decide which papers to review? As a junior academic I am beginning to get requests to review papers.

What standards do you people out there use to decide which review requests to accept and which to turn down?
I've found that after you get a few decent papers, you'll soon get far more requests than you can possibly handle (e.g. over the last few years I've consistently received one every week or two). 

Besides the obvious issues of whether you're competent AND interested in reading the paper AND are currently available, my calculus usually goes like this: 


Is this a very high end journal? E.g. in my field that would be JAMA, NEJM, Science, etc. Accept the request. It's good to have input on some of the most influential papers in your field. 
If not, is it a journal you've never reviewed for before? It's always good to be able to add to the list of journals on your CV. If I haven't, but I HAVE heard of the journal (e.g. I know it's not a predatory journal), I'll usually accept. 
Is it an editor you have a history with/have a decent reviewer/editor relationship with? Accept the request. It's good to have arms-length senior academics who like you and are aware that you exist and know your expertise--e.g. for external non-collaborator letters when you're up for tenure. 


If the answer to all three of those is "No", I'll usually decline and suggest another colleague.