Knowing where you are going ahead of time is always a good idea, and riding Metro is no exception. Know where you are starting, where you are exiting, and what lines and transfers you'll need before you start your trip. Most directional labels in the Metro system refer to the last station on a line in a given direction. If you are planning to ride the Orange or Blue line from Rosslyn to Smithsonian, for example, you are headed in the direction of New Carrollton (the eastern most station of the Orange line) and Largo (the eastern most station of the Blue line).
The traditional metro map is a depiction of relative location, but it does not always show accurate distances. While it probably will not change your travel route, it is worth considering if you are trying to get a sense of how long a trip will take. Often, stations the National Mall have more separation that those in downtown DC, but this is not apparent by looking at the DC Metro map.