Throughout human history people have expressed themselves through singing – whether celebrating victory in battle or comforting each other through the oppression of slavery. The New Testament tells God's people to “be filled with the Spirit, singing and psalming with your hearts to the Lord, speaking to one another in psalms and hymns and spiritual songs” (Eph 5:18-19) – a picture of corporate music-making that has both vertical and horizontal dimensions. Leading the gathered church in song is an important act of service.
If the service leader or preacher has introduced the song, there's no need to say any more
About two sentences is generally the right length
Plan each sentence in your head before saying anything – ums and ahs waste the congregation's time and give them an opportunity to lose focus.
The basic “Please stand and we'll sing together” is a good start
Introduce what the big point of the song is – generally what the chorus is about – rather than picking out some line from the second verse (unless that line is really really significant)
When linking songs, think about what the main point of the previous song was, what the main point of the next song is, and what the theological link between them is.
Introducing the song after the sermon is a key moment and you need to start planning when the end of the sermon is coming near. You want to make a connection, but without extending the sermon, twisting its message in a new direction or offering a commentary on it. Is there a key phrase you can repeat and link to the song? Or something in the bible passage that links? Occasionally the sermon will end up somewhere unexpected and the song won't connect closely at all – in this case an intro that just talks about the song without mentioning the sermon is better than one that makes a really contrived link.
Personal reflections can sometimes be beneficial (generally not after the sermon) but be careful that the focus ends up on Jesus and his church rather than on you.
Be deliberate with your body language. If you take a breath in an obvious way people will realise we're about to sing. If your hands are in your pockets people will realise you're bored. If you look at the lyrics the whole time people will think you don't know what you're singing and don't really mean it.
Respond to what the song is saying – there's a time for beaming with joy, but it's not when you're singing about the details of the crucifixion.
When we're singing to each other (eg “Behold the Lamb of God who takes away our sin”) or about to start a verse, make eye contact with the congregation. When we're singing to God (eg “Jesus, I trust in you”), focusing on a point slightly above the congregation is more natural. Closing your eyes separates you from the congregation and stops you from leading properly.
Gestures to indicate entries or quick turnarounds are helpful
Remember the congregation will be no more enthusiastic than you look...