On Sunday I was coordinating the service at our church (Auckland Chinese Presbyterian Church) and I offered a couple of thoughts on worship, in particular, singing.
One bad habit it's easy to fall into is to sing along with everyone else without thinking about the words. If we're going to be honest with God, ourselves, and each other, we need to mean what we sing and sing what we mean. If the words say something we don't agree with, we shouldn't sing them. If I have an atheist friend at church and they don't sing anything, I respect that.
On the other hand, some Christians don't sing because they just don't feel like it --- perhaps they're having a bad day, or a bad year. But worship is not supposed to depend on how we feel; it's something we're commanded to do no matter how we feel. If a song is true then there's no harm and some benefit to singing it. (Of course singing is not the only form of worship, and if someone has taken a vow of silence, I respect that too :-).)