When I was at university I found most of my lectures very interesting and did pay full attention most of the time. There were the occasional ones that were less interesting, and as a result my mind would drift off more. Most of my lectures were about 3 hours long (but with a 10-15 minute break in the middle) so sometimes I would lose concentration in the last 15 minutes or so.
Generally I found taking notes helped me pay attention. We were always given hand outs, so I would annotate these. I didn't write word for word what the lecturer was saying, but instead summarised things, labelled graphs, diagrams etc.