Let's get straight to it, shall we?
You have to learn to count beats in music.
There, I said it. Disagree? Leave a comment below.
Now, let's be clear here, I didn't say you have to learn all the numbers and names of the subdivisions, though it would help. You just have to count, or perhaps account for, the beats as they pass in the music.
While there are many reasons for this, the overwhelming advantage to the vocalist who does this is that they will know exactly, with no doubt whatsoever, not only when their entrance is, but how much time they have until they get to it. This directly translates into preparedness for the phrase, and that is paramount in allowing us to give our best performances.
For many of us, this directly translates into confidence on stage. There are two aspects to this, both of which are important. First of all, knowing where one's entrance is in and of itself can be very helpful in developing confidence. Second, if you're busy counting beats in your mind, you don't have the time to get distracted with negative thoughts or concerns. Counting music, in my opinion, actually brings you closer to it, embedding you in the making of it when you're actively participating (by singing, for example) and when you're are passively participating (counting rests).
Even the most basic of beat counting can have a dramatic impact on your understanding of pulse and rhythm, and consequently transform your understanding of the music that you're making. It's a small practice to include, for such great rewards.