Pace your song. Keep things moving smoothly by using repetition, variation, and interruption.
Pacing means managing the overall shape of the song. The song is an experience that unfolds over a period of time (a few minutes), and you should make sure that it unfolds in a smooth and graceful way. For the sake of good pacing, sometimes you'll want to move things along quickly, and find ways to speed things up. But there will also be times when you'll want to bring things to a stop or keep things static, in order to punctuate and experience or deepen an element of the song.
You have to be able to experience your own song as a listener. That's the only way you can know when the pacing is right and when you need to make adjustments. This means you can't afford to be stuck in the role of creator. Learn to shift quickly and easily from (proud, willful) creator to (detached, objective) listener, so that you can check your progress and know whether you're keeping the song on track.
You're the tour guide for your audience, and that means it's your job to know the terrain. Never take the listener to the same place twice. If you do, they'll think you've gotten lost, and then they will lose confidence in you. Make sure you earn their confidence, and make sure you reward them for sticking with you.