Here are some video tutorials with tips you may use while sketching on location.
This first video is a reply to a common response that I get from people who say they can't sketch because they don't know perspective.
You can use the techniques of creating a strong silhouette to improve the clarity of your sketch
Someone said he/she could draw small objects but not big buildings. You can by starting with the general shape of the building first.
This is a technique to improve depth in your sketch