When we work on songs, it's easy to get overwhelmed without realizing that we're getting overwhelmed. Even the simplest of tunes is far more complex than the scales that we sing in our vocalizations. In order to have our vocalizations help our songs the most, we do well to create vocal exercises out of the melodies of our songs, or find warm ups that are as similar as can be. If we break this down into separate concepts, it becomes more easy to manage.
To effectively warm up for a song, we have to be certain to cover the range that is spanned in the song. This means knowing what the lowest and highest notes are. While vocalizing to the lowest note is fine, it's best to vocalize above the highest note by as much as a minor third. More than that is fine for overall technical practice, but isn't necessary if we're just warming up for this one song.
Warm ups are generally in a major tonality so that we can more easily predict the next pitch that we are going to sing. Songs, of course, are not always like that. Examine the melodic content of your song; does it have minor elements, large skips, unusual turns of phrases? Those individual elements may need to be worked on separately before being reintegrated into the performance practice of the song (more on that below).
Does the song require you to sing particularly loud? If so, then it is also asking you to sing very balanced and soft as well. Dynamics are relative. Not absolute. One vocalist will be able to sing fortissimo objectively louder than another based upon the configuration and usage of their instrument (their voice!). Both vocalists, however, can maximize the dynamic contrast available to them by practicing balanced, low volume, yet viable sounds. In other words, creating a low volume, but otherwise consistent, well in-tune sound to contrast with the louder dynamics available to them.
Memorize the Song
There's a reason why hands free devices are generally required to talk on the phone while driving. Even that, however, I have to admit is very distracting! More than just being less distracted, however, memorizing your songs allows you to predict what is coming far more effectively than if you're reading off of a sheet. Really *knowing* the song frees you to bring your absolute best to the performance
Separate Performance Practice from Practice
Performance practice is a separate practice than practicing a song. When we're learning, or adapting our technique to a song, stopping frequently when something has gone wrong is expected and to be encouraged. We may find that the chorus needs more vocalizing in order to get the range sorted out, or a particular sequence of words has the verse sounding too disjointed. When we're practicing, we stop to work on those elements. Having done that, we can move on to performance practice, where we run through the song in its entirety. We may not do that every time we work on the song. We may only do it when we're in the process of getting ready to perform. It will almost certainly make us aware of other parts of the song that need more practice. This is a big part of how we grow as artists, and develop the kind of consistency in performance that is expected of professional musicians.