If I came to your church tomorrow, I could put together two different worship services, and one would have more participation than the other. If I knew a little about your churches background, and had a songlist of the songs your people do, I could plan the two services so that one would have people singing out loud and the other, they would stand there frustrated and lost. You know what the answer is. You know how I could do this, and yet each week worship leaders around the country face this very problem.
Familiarity is the key to good worship participation.
If you don't believe me, try throwing together songs that no one knows during a service. See how much singing and participation there is. Then, try using songs that everyone knows and you will see the difference. Simple, and yet I am guilty of it on a weekly basis. Partly because I want to sing the new songs.