How to concentrate during conference talks where the quality of the presentation is poor?
When I attend talks where the quality of the presentation is poor, I like to use it as an opportunity to learn about what not to do in a presentation.

I have an ongoing list of "things not to do in a presentation", with examples from talks I've attended. I just take out the list and add to it whenever I attend one of these presentations. I find it helpful for improving the quality of my own presentations - more helpful than just reading tips others have written about presenting, because when I refer to my list I am reminded of exactly why and how a presentation is made worse by [insert bad thing here], from personal experience.

Usually as a result of taking notes on the presentation technique, I end up also paying attention to the content of the presentation, which is a nice side benefit.

P.S. I also have a list of "effective presentation techniques" and I add to it whenever I attend a talk where the quality of presentation is very good.