A natural follow-up to last week's tip is to look for shadows.   The first tip is to notice shadows like you notice light.  Is the shadow harsh with defined lines or is it a subtle diffused shadow?  Another aspect of shadows that has caused me issues is how the shadows are hitting my subject.  I have taken several photos that have unwanted shadows across my subject that I didn't notice until I got home.

What I have learned here is that shadows need to be observed and understand how they will be interacting with my photo.