Something I find interesting is to watch students watching an instructor demonstrating technique. It amazes me how often people are looking around the gym (what is there to look at??) or just aimlessly staring at some random point. Even some of the people who are watching the technique are clearly not watching properly.
The most important thing to realise is that everything the instructor is doing is important to the technique. In every position and during every transition everything matters; head, arm & leg positioning, hip angle, weight distribution, grips etc... absolutely everything. So don't just look at the general things which are happening, concentrate on all the little things and try to understand why your instructor is doing it (understanding principles is much better than simply copying without knowing why). Every time they run through the technique, try to pick out different things that are going on.
Also, make it an interactive experience. Don't just sit in one spot if there is stuff going on which you can't see. Watch the technique through from one angle and then the next time the instructor runs through it move to the other side. Ask questions; about things that you don't know why it matters (i.e. "Is there a reason you're putting your foot there?") or anything you're unsure of (i.e. "Do you have to use that grip?"), but avoid questions which are outside the scope of the technique.
Lastly, this should go without saying but it doesn't always... don't talk to other people while you're watching the technique demo or do anything else which will distract people. You're there to learn jiu-jitsu, learning anything takes effort and you will only get out what you put in, so give it your full concentration and effort!