Note what works & what doesn't work
When using visuals, take note of the expression you see on people's faces when they see the slide. If they are confused, it may be a signal that the slide is not designed as clearly as you thought it was. This doesn't mean that you should necessarily go and change the slide for your next presentation. The benefit of noting it in a journal is that you can see patterns emerge.
If you note that a slide didn't work well and see in your journal that this is the third note about that slide, it is a trend that suggests that the issue is with the slide, not the audience. After you make a change to a slide, track whether the new slide is working or whether further tweaks are necessary. Use the trends that appear as a more reliable indicator than the reactions of a single audience. You will end up with a better reading that way.
Use Darci's suggestion of journaling what works and what doesn't work with your slides and you will see a big improvement in 2009.