In a discussion of media bias and regulation on another sub, I found myself tempted to point out that America had solved this problem before...but I realized that I had no idea how, or even if it truly had. Casual searches make it appear as though Hearst and Pulitzer simply developed a conscience and changed their ways after McKinley was shot. But I'm skeptical that they both just had Ebenezer Scrooge transformations and thus ended an ugly era. How did that period of journalism come to an end...if it did?
Starting around 1902, various Progressive activists and writers tried to make journalism respectable and the newspaper trades became increasingly professionalized. This process accelerated after 1908 as the first formal "journalism school" was founded at University of Missouri. By 1917, the sensationalisitc and muckracking styles of newpaper writing were falling out of favor, although I'm sure the war effort helped with this somewhat. During the 1920s and 1930s journalism became increasingly professionalized, and by the 1950s many (if not most) professional newspaper journalists are graduates of collegiate journalism schools. J-schools tended to teach a very specific style of writing narrative which is often presented (somewhat misleadingly) as "neutral".
For further reading, I suggest Journalism, 1908: Birth of a Profession as a good scholarly source for current understanding on the changes that newspapers and the people writing for them underwent in the early 20th century.