How to format for a useful journal club?
This is based on my experience being in some highly unsuccessful journals clubs, and some very successful ones - at least in my mind.


You must have faculty involvement. I've seen more than one journal club that either didn't have faculty members, or had a faculty member or two who just kind of sat back and didn't say anything. That's bad. Faculty members who can contribute, answer questions, and generally provide some context for papers are excellent. They're good for pointed questions we might have missed - I've had faculty members ask a question about a figure that got into an interesting discussion of research ethics, one that led insight into some politics ("The reason that commentary appeared in this journal is Y"), etc.
I prefer to have them separate from lab meetings, and drawing from a wider audience than my specific research group. I find the breadth of experience, diversity of papers, and keeping up with things taking place beyond my narrow little laser-like focus to be both refreshing and more useful than going over a paper half of us already read.
Giving the journal club a greater context. Yes, keeping track of the literature is important. But its importance seems to slide if you know your analysis should be done soon, or something needs to come out of the water bath, or midterms need to be graded. One semester we framed ours as qualifying exam preparation, and another as professional development - the people presenting wrote their critiques like responses to requests for peer review.


Overall, I've found journal clubs to be most useful for mid-level graduate students - they need enough experience to have thoughts, insights and feelings about the paper, but if a JC succeeds, eventually they should need it less and less.