Write more than you need, and use less than you write.
Don't stop when you have enough verses, or enough lines. Keep writing! Write more, and keep writing until you run out of ideas and inspiration. Sometimes the most surprising ideas slip out after you think you're already done, and after the pressure is off. Your song might take an unexpected change of direction that turns it from good to brilliant.
Later, in another writing session, select the best lines or the best verses, and ruthlessly excise all the others. It can be painful to chop out a verse that's perfectly good, or an exquisite rhyming joke, but remember that there is something more painful: performing for a restless audience who wishes your song was already over.
Write a lot, but use just enough, and no more.