Making the best use of "time" and "intros" in your songs.

Time in a song is pricey real estate, therefore use it well.  Just like in any conversation, a "pitch" line right at the beginning could mean hitting the ON or OFF button, the same happens musically.

For example, it has been researched that 3/4 time tends to actively work on the brain in a different way than 4/4 time. Therefore, if your song is 3/4, 6/8, or any of the compound meters, your listener will remember something about its rhythmic pattern that will tend to stay longer in their mind.  The brain may say: "this is unusual..."

Therefore, if your song (like the majority in the world today) is in simple meter or 4/4 time, make the best use of your intro.  However, the same principle should be said for compound meter.  Making the best use of the intro is always a good practice.  If not sure of the intro, leave it for last.  This technique has proven effective in speeches, use it in a musical way and it will become another goodie for the bag of tricks.

How to make good use of the intro and its timing?  Look for patterns that make contrary motion with vocals.  Example:  one repetitive chord over an active vocal line (or vise versa).  This specific example, by the way, will make your melodic line in the vocals "free-up" some of its own artistic tendencies.  Sometimes this is used in songs in order to let the melody survive the battle of harmony vs. melody.

However, these are just tips in order to achieve a purpose.  Feel free to experiment and move boundaries.