The August 11, 1928 issue of The New Yorker had an interesting article about Tin Pan Alley songwriters.
Apparently, songwriters bore the responsibility of paying influential people to "plug" their songs. These people -- called "angles" -- tended to be radio singers who were allowed to choose the songs they wanted to perform; if a songwriter offered them thirty to fifty percent of the song's royalties, the radio singers would sing it.
Likewise, a songwriter could end up paying money to additional "angles" such as "jazz-band leaders, makers of phonograph records, movie-house program directors, and the like."
It's interesting that -- according to the article -- the music publishers used to be responsible for paying the angles, at that time usually vaudeville singers. But after theatre managers complained that the vaudeville shows were suffering from substandard material and constant repetition, the music publishers formed the Music Publishers Protective Association and vowed to stop the practice...which is why, by the 1920s, it was up to the writer to do the dirty work.
Compare this to the more modern practice of payola, in which music publishers get back into the game and pay DJs to play their songs.
Incidentally the article concludes by saying there were few successful female songwriters, and that the often-seen name "Mary Earl" was actually "a copyrighted pseudonym belonging to a firm of music publishers and is used on songs written by men."