When sketching landscapes quickly, it is important to focus on the essential elements and not get bogged down in details. One strategy is to use gestural lines to quickly capture the overall shape and composition of the landscape. Another strategy is to use value sketches, which involve quickly blocking in the dark and light areas of the landscape to establish the overall tonal range. You can also use a viewfinder to help you focus on a specific area of the landscape and avoid getting overwhelmed by the entire scene.