Writing an Effective Melody
Of all the aspects involved with creating new songs, working on writing a strong melody is undoubtedly the most commonly overlooked in modern pop/rock music.
This wasn't always the case; the "pop" songwriters of the 1930's and 1940's focused greatly on writing melodies. In many cases the melody was the basis for a song, with lyrics and chords added later.
Generally, the process of writing a song is much different nowadays. Often, songs will be born out of a guitar riff, or a groove. This is built upon, and a chorus is written, basslines added, etc., so that the entire instrumental part of the song has been assembled even before the melody has been considered. From my experience in watching many bands undertake the process of writing music, the vocal melody will often then be added quickly, almost without thought. This isn't the best approach - without a strong melody, the vast majority of people won't give a song a second thought.