A master's degree can definitely improve your salary. It is something that you should achieve while working if you want to have the best impact to your salary. It seems that going straight from high school to a master's degree with no job experience is less beneficial. Either way the more education you have the better for salary.
Generally speaking, master's degrees increase the earning potential of their recipients. This is particularly true in hard science and engineering disciplines. Masters degrees in liberal arts may be less lucrative without some associated professional school, such as law school, but recipients of liberal arts masters likely make more in their professions than those without them.